file-wmf - allow to redefine WMF font directory using WMF_FONTDIR env variable
While working on gimp/osx i found that wmf import is not working. During debugging i found that it always trying to use path compiled in the libwmf, what is usually %PREFIX% defined during configuration, e.g. /usr/share/libwmf. This patch allows to redefine it if appropriate variable is set. If it is not - old logic is in use.
Patch is tested and proved to work on my OSX build.