@ollydbg: Thanks for your comment.
It appears that either mingw32-make.exe or makefile.gcc is exiting prematurely without any error message.
I've used the following command: mingw32-make.exe -f makefile.gcc CFG=64 BUILD=Release SHARED=1 MONOLITHIC=1 >out10 2>err10 at a command prompt for awhile (years) without any problems.
Now I would end-up with approximately 15 subroutines and wxmsw32u_gcc_custom.dll.rsp in the gcc_dll64 library. This last file is usually a dll of approx.27,000+Kb is size, but now is only 25+Kb. There also is usually a second dll file.
Regards