Since fontconfig didn't have special handling for paths in static Windows
libraries, I've created a patch which should fix this.
Basically it does this:
fccfg.c:
If fontconfig_path was uninitialised it tries to get the directory the exe is
in and uses a fonts/ dir inside that.
fcxml.c:
In case the fonts.conf lists a <dir>CUSTOMFONTDIR</dir>, it searches for a
fonts/ directory where the exe is located.