Fixed broken execute_process() call under GNUstep shell on Windows

[ElMostafaIdrassi]

For gnustep-config bash script to be invoked properly by execute_process() under GNUstep shell on Windows, the ${GNUstepConfig} call must preceded by sh. Otherwise, execute_process() fails, leading to objc_def_flags and objc_def_link_flags being empty, with RESULT_VARIABLE containing : %1 is not a valid win32 application.

[mulle-nat]

The existing GNUstep code was written by someone else, so I can only guess if yours is correct or not (same for the old GNUstep code though). I don't want all that text in the README.md though. I'd put that into a separate BUILD-GNUSTEP-WINDOWS.md if that's OK with you,

[ElMostafaIdrassi]

Well you can give it a try if you have some time, without adding sh in the execute_process(), the compiler and linker flags are empty on Windows, you can print them using message( STATUS...), and without the manual trick, the build fails.

Maybe you should add a note that this was the behaviour using GNUstep shell from MSYS version 0.30 downloaded from here and it comes with GCC 4.6.1.

For the Windows build steps, feel free to manage that as you wish ;)

Cheers

[mulle-nat]

I will probably put up a new version of this and some other changes this week.