wxwidgets ships two copies of that header file, one for 32-bit compiles and one for 64-bit compiles.
If you run "diff -u /usr/lib/wx/include/gtk2-unicode-3.0/wx/setup.h /usr/lib/64/wx/include/gtk2-unicode-3.0/wx/setup.h" you'll see whether or not wxSIZE_T_IS_UINT is defined depends on whether you're using the 32-bit or 64-bit header.
Running "/usr/bin/wx-config --cflags" produces the -I path to the 32-bit header, while "/usr/bin/64/wx-config --cflags" gives the -I path to the 64-bit header.
Did you perhaps use the 32-bit path when building a 64-bit binary?
Not the first time I have made that error. Adding "/usr/bin/64" to my PATH made sure the configure picked the right binaries.
I was building filezilla and it builds on Solaris 11.4 so thanks.