Setting it in config/win32.cbc and win64.cbc means that it doesn't get
set when you run cerbero without any -c arguments, which is the most
common mode.

win32/win64.cbc are only used while generating the gstreamer binary
releases.

If we don't disable parallel build for make, we get errors like:

C:\MinGW\msys\1.0\bin\make.exe: *** couldn't commit memory for cygwin heap, Win32 error 0