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.
Minor enhancements to the MinGW+Msys build scripts.

Makefile now creates a zipfile containing the DLLs.

Resulting DLL no longer contains a version number in its name.  (The
Windows convention is to ship the DLL with the program using it.
Versioning the name just makes it harder for scripts to find the DLL.)

Tweaked the README a bit.