Updated .gitignore.

Now ignores all (so far) of the products of the MSys+Windows build
process.
Got gd_filename.c to work correctly under Windows.

Specified calling convention on function pointers.  (This matters
under Windows).

Switched from rindex() to strrchr() (oops!).

Tidied the formatting a bit.
Updated the MSys build files.

The Makefiles and scripts for building under MSYS had rotted a bit.  This
change fixes those things.
Merged from the upstream.

Specifically, 'master' of https://bitbucket.org/libgd/gd-libgd