[Mingw-cross-env-list] gd: define required for static linking (NONDLL)

[Natalie Tasman]

I'm getting link errors with gd which look like dynamic
implementations are expect for the static library.

I checked i686-pc-mingw32-pkg-config, but it seems that gd doesn't use
packageconfig (no corresponding .pc file.)   So I looked in the gd.h
header for anything about "static", and found the "NONDLL" define
(-DNONDLL flag) which indeed fixes the problem when added to my own
program's build.

I'm not sure how this should be fixed in mingw-cross-env, so I'm
posting here to at least share my experiences.  Perhaps a "sed fix" is
in order for the installed gd.h header in mingw-cross-env.

Best,

Natalie