build: Conditionally add gwin32.c to sources without using EXTRA_ prefix
Instead of messing around with EXTRA_*_SOURCES and manually handling .lo
files, why not just add gwin32.c to the GLib sources conditionally?
This will hopefully fix `make distcheck` failing due to gwin32.Plo not
being generated in the sub-builddir≠srcdir stage, due to depcomp
inexplicably not generating it. (Note that it is correctly generated in
non-distcheck builds.)
Signed-off-by: Philip Withnall <withnall@endlessm.com>