The cstddef discovery is from https://stackoverflow.com/questions/52567517/freefilesync-c-error-byte-is-not-a-member-of-std
Some changes just make the application work better in an installed setting (e.g., the hard-coded resouce path).
Tricking the wxWidgets ABI version comes from https://stackoverflow.com/questions/45123664/wxwidgets-runtime-error-mismatch-version/47822976#47822976
FreeFileSync 10.11 updated the default compile flags to -std=c++2a, but this fails on gcc 4.8.5 on el7. Reverting to -std=c++17 allows the application to compile.
diff -Naur 10.13-0/FreeFileSync/Source/base/ffs_paths.cpp 10.13-1/FreeFileSync/Source/base/ffs_paths.cpp
--- 10.13-0/FreeFileSync/Source/base/ffs_paths.cpp	2019-06-14 20:23:07.615661499 -0400
+++ 10.13-1/FreeFileSync/Source/base/ffs_paths.cpp	2019-06-14 20:36:28.178274292 -0400
@@ -55,7 +55,8 @@
 
 Zstring fff::getResourceDirPf()
 {
-    return getProcessParentFolderPath() + FILE_NAME_SEPARATOR + Zstr("Resources") + FILE_NAME_SEPARATOR;
+    // For Fedora, EL and Devuan, install to /usr/share/freefilesync specificly
+    return Zstr("/usr/share/freefilesync/");
 }
 
 
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.11-0/FreeFileSync/Source/Makefile 10.11-1/FreeFileSync/Source/Makefile
--- 10.11-0/FreeFileSync/Source/Makefile	2019-04-12 07:59:45.042147902 -0400
+++ 10.15-1/FreeFileSync/Source/Makefile        2019-08-16 08:19:12.101271747 -0400
@@ -1,10 +1,10 @@
-EXENAME = FreeFileSync_$(shell arch)
+EXENAME = FreeFileSync

-CXXFLAGS = -std=c++2a -pipe -DWXINTL_NO_GETTEXT_MACRO -I../.. -I../../zenXml -include "zen/i18n.h" -include "zen/warn_static.h" \
+CXXFLAGS = -std=c++17 -pipe -DWXINTL_NO_GETTEXT_MACRO -I../.. -I../../zenXml -include "zen/i18n.h" -include "zen/warn_static.h" \
            -Wall -Wfatal-errors -Wmissing-include-dirs -Wswitch-enum -Wcast-align -Wshadow -Wnon-virtual-dtor \
-           -O3 -DNDEBUG `wx-config --cxxflags --debug=no` -pthread
+           -O3 -DNDEBUG -fPIC `wx-config --version=3.0 --cxxflags --debug=no` -pthread

-LINKFLAGS = -s -no-pie `wx-config --libs std, aui --debug=no` -pthread
+LINKFLAGS = -s -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -Wl,-z,relro  -no-pie `wx-config --version=3.0 --libs std, aui --debug=no | sed -r -e 's/-[^[:space:]]+web[^[:space:]]+//;'` -lz -pthread


 CXXFLAGS  += `pkg-config --cflags openssl`
@@ -17,6 +17,7 @@
 LINKFLAGS += `pkg-config --libs   libssh2`

 CXXFLAGS  += `pkg-config --cflags gtk+-2.0`
+LINKFLAGS += `pkg-config --libs   gtk+-2.0`
 #treat as system headers so that warnings are hidden:
 CXXFLAGS  += -isystem/usr/include/gtk-2.0

@@ -56,7 +56,6 @@
 CPP_FILES+=afs/init_curl_libssh2.cpp
 CPP_FILES+=afs/native.cpp
 CPP_FILES+=afs/sftp.cpp
-CPP_FILES+=afs/libssh2/init_libssh2.cpp
 CPP_FILES+=ui/batch_config.cpp
 CPP_FILES+=ui/abstract_folder_picker.cpp
 CPP_FILES+=ui/batch_status_handler.cpp
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.11-0/FreeFileSync/Source/RealTimeSync/application.h 10.11-1/FreeFileSync/Source/RealTimeSync/application.h
--- 10.11-0/FreeFileSync/Source/RealTimeSync/application.h	2019-04-12 07:59:45.035147580 -0400
+++ 10.11-1/FreeFileSync/Source/RealTimeSync/application.h	2019-04-12 10:19:57.227913415 -0400
@@ -7,6 +7,8 @@
 #ifndef APPLICATION_H_18506781708176342677
 #define APPLICATION_H_18506781708176342677
 
+#define __GXX_ABI_VERSION 1002
+
 #include <wx/app.h>
 
 
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.11-0/FreeFileSync/Source/RealTimeSync/Makefile 10.11-1/FreeFileSync/Source/RealTimeSync/Makefile
--- 10.11-0/FreeFileSync/Source/RealTimeSync/Makefile	2019-04-12 07:59:45.008146336 -0400
+++ 10.15-1/FreeFileSync/Source/RealTimeSync/Makefile   2019-08-16 08:19:21.959638104 -0400
@@ -1,13 +1,14 @@
-EXENAME = RealTimeSync_$(shell arch)
+EXENAME = RealTimeSync

-CXXFLAGS = -std=c++2a -pipe -DWXINTL_NO_GETTEXT_MACRO -I../../.. -I../../../zenXml -include "zen/i18n.h" -include "zen/warn_static.h" \
+CXXFLAGS = -std=c++17 -pipe -DWXINTL_NO_GETTEXT_MACRO -I../../.. -I../../../zenXml -include "zen/i18n.h" -include "zen/warn_static.h" \
            -Wall -Wfatal-errors -Wmissing-include-dirs -Wswitch-enum -Wcast-align -Wshadow -Wnon-virtual-dtor \
-           -O3 -DNDEBUG `wx-config --cxxflags --debug=no` -pthread
+           -O3 -DNDEBUG -fPIC `wx-config --version=3.0 --cxxflags --debug=no` -pthread

-LINKFLAGS = -s -no-pie `wx-config --libs std, aui --debug=no` -pthread
+LINKFLAGS = -s -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -no-pie `wx-config --version=3.0 --cxxflags --libs std, aui --debug=no | sed -r -e 's/-[^[:space:]]+web[^[:space:]]+//;'` -lz -pthread

 #Gtk - support "no button border"
 CXXFLAGS  += `pkg-config --cflags gtk+-2.0`
+LINKFLAGS += `pkg-config --libs   gtk+-2.0`
 #treat as system headers so that warnings are hidden:
 CXXFLAGS  += -isystem/usr/include/gtk-2.0

diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.11-0/FreeFileSync/Source/ui/small_dlgs.cpp 10.11-1/FreeFileSync/Source/ui/small_dlgs.cpp
--- 10.11-0/FreeFileSync/Source/ui/small_dlgs.cpp	2019-04-12 07:59:45.318160616 -0400
+++ 10.11-1/FreeFileSync/Source/ui/small_dlgs.cpp	2019-04-12 10:19:57.228913439 -0400
@@ -115,6 +115,8 @@
         L" x64";
 #endif
 
+    build +=
+        L" for CentOS 7";
 
     GetSizer()->SetSizeHints(this); //~=Fit() + SetMinSize()
 
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.11-0/wx+/app_main.h 10.11-1/wx+/app_main.h
--- 10.11-0/wx+/app_main.h	2019-04-12 07:59:44.936143019 -0400
+++ 10.11-1/wx+/app_main.h	2019-04-12 10:19:57.228913439 -0400
@@ -7,6 +7,8 @@
 #ifndef APP_MAIN_H_08215601837818347575856
 #define APP_MAIN_H_08215601837818347575856
 
+#define __GXX_ABI_VERSION 1002
+
 #include <wx/window.h>
 #include <wx/app.h>
 
Version: FreeFileSync 10.17
Distro: EL7,EL8,fc29
Message: short-circuit for EL and Fedora 29. This probably has to do with gcc < 9. Conveniently, x86 and x86_64 are my only targets and they are little-endian.
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.17-0/zen/build_info.h 10.17-1/zen/build_info.h
--- 10.17-0/zen/build_info.h	2019-10-21 11:23:51.249139703 -0400
+++ 10.17-1/zen/build_info.h	2019-10-22 10:52:41.846538926 -0400
@@ -24,7 +24,9 @@
 
 constexpr bool usingLittleEndian()
 {
-    return std::endian::native == std::endian::little;
+    return true;
 }
 
 #endif //BUILD_INFO_H_5928539285603428657
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.11-0/zen/ring_buffer.h 10.11-1/zen/ring_buffer.h
--- 10.11-0/zen/ring_buffer.h	2019-04-12 07:59:46.098196548 -0400
+++ 10.11-1/zen/ring_buffer.h	2019-04-12 10:19:57.228913439 -0400
@@ -9,6 +9,7 @@
 
 #include <cassert>
 #include "scope_guard.h"
+#include <cstddef>
 
 
 namespace zen
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.11-0/zen/serialize.h 10.11-1/zen/serialize.h
--- 10.11-0/zen/serialize.h	2019-04-12 07:59:46.080195719 -0400
+++ 10.11-1/zen/serialize.h	2019-04-12 10:19:57.229913463 -0400
@@ -9,6 +9,7 @@
 
 #include <functional>
 #include <cstdint>
+#include <cstddef>
 #include "string_base.h"
 //keep header clean from specific stream implementations! (e.g.file_io.h)! used by abstract.h!