FcStrCanonFileName buggy for mingw. (bug 8311)

FcStrCanonFileName checks whether s[0] == '/', and recurses if not.

This only works on POSIX. On dos, this crashes with a stack overflow.

The patch attached splits this functionality in two functions
(FcStrCanonAbsoluteFilename) and uses GetFullPathName on windows to get an
absolute path.  It also fixes a number of other issues. With this patch,
LilyPond actually produces output on Windows.