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
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.9-0/FreeFileSync/Source/base/ffs_paths.cpp 10.9-1.el7/FreeFileSync/Source/base/ffs_paths.cpp
--- 10.9-0/FreeFileSync/Source/base/ffs_paths.cpp	2019-02-13 16:05:15.000000000 -0500
+++ 10.9-1.el7/FreeFileSync/Source/base/ffs_paths.cpp	2019-02-17 10:43:22.286986341 -0500
@@ -59,7 +59,7 @@
     ZEN_ON_SCOPE_EXIT(wxTheApp->SetAppName(appName));
 
     //if (isPortableVersion())
-    return appendSeparator(getExeFolderParentPath());
+    return appendSeparator("/usr") + appendSeparator("share") + appendSeparator("freefilesync");
     //else //use OS' standard paths
     //    return appendSeparator(utfTo<Zstring>(wxStandardPathsBase::Get().GetResourcesDir()));
 }
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.9-0/FreeFileSync/Source/Makefile 10.9-1.el7/FreeFileSync/Source/Makefile
--- 10.9-0/FreeFileSync/Source/Makefile	2019-02-13 16:05:15.000000000 -0500
+++ 10.9-1.el7/FreeFileSync/Source/Makefile	2019-02-17 10:50:27.282820639 -0500
@@ -1,10 +1,10 @@
-EXENAME = FreeFileSync_$(shell arch)
+EXENAME = FreeFileSync
 
 CXXFLAGS = -std=c++17 -pipe -DWXINTL_NO_GETTEXT_MACRO -DLIBSSH2_OPENSSL -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`
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.9-0/FreeFileSync/Source/RealTimeSync/Makefile 10.9-1.el7/FreeFileSync/Source/RealTimeSync/Makefile
--- 10.9-0/FreeFileSync/Source/RealTimeSync/Makefile	2019-02-13 16:05:15.000000000 -0500
+++ 10.9-1.el7/FreeFileSync/Source/RealTimeSync/Makefile	2019-02-17 10:50:43.699007372 -0500
@@ -1,10 +1,10 @@
-EXENAME = RealTimeSync_$(shell arch)
+EXENAME = RealTimeSync
 
 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`
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.9-0/FreeFileSync/Source/ui/small_dlgs.cpp 10.9-1.el7/FreeFileSync/Source/ui/small_dlgs.cpp
--- 10.9-0/FreeFileSync/Source/ui/small_dlgs.cpp	2019-02-13 16:05:15.000000000 -0500
+++ 10.9-1.el7/FreeFileSync/Source/ui/small_dlgs.cpp	2019-02-17 10:42:29.741388638 -0500
@@ -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' 10.9-0/zen/ring_buffer.h 10.9-1.el7/zen/ring_buffer.h
--- 10.9-0/zen/ring_buffer.h	2019-02-13 16:05:15.000000000 -0500
+++ 10.9-1.el7/zen/ring_buffer.h	2019-02-17 09:53:10.907818287 -0500
@@ -9,6 +9,7 @@
 
 #include <cassert>
 #include "scope_guard.h"
+#include <cstddef>
 
 
 namespace zen
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.9-0/zen/serialize.h 10.9-1.el7/zen/serialize.h
--- 10.9-0/zen/serialize.h	2019-02-13 16:05:15.000000000 -0500
+++ 10.9-1.el7/zen/serialize.h	2019-02-17 09:53:06.701772057 -0500
@@ -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!
 
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.9-0/wx+/app_main.h 10.9-1/wx+/app_main.h
--- 10.9-0/wx+/app_main.h	2019-02-13 16:05:15.000000000 -0500
+++ 10.9-1/wx+/app_main.h	2019-02-16 13:51:32.437266070 -0500
@@ -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>
 
diff -Naur -x '*.orig' -x '*.rej' -x '*.swp' 10.9-0/FreeFileSync/Source/RealTimeSync/application.h 10.9-1.el7/FreeFileSync/Source/RealTimeSync/application.h 
--- 10.9-0/FreeFileSync/Source/RealTimeSync/application.h	2019-02-13 16:05:15.000000000 -0500
+++ 10.9-1.el7/FreeFileSync/Source/RealTimeSync/application.h	2019-02-18 08:53:14.017850931 -0500
@@ -7,6 +7,8 @@
 #ifndef APPLICATION_H_18506781708176342677
 #define APPLICATION_H_18506781708176342677
 
+#define __GXX_ABI_VERSION 1002
+
 #include <wx/app.h>