file-wmf - allow to redefine WMF font directory using WMF_FONTDIR env variable

Alex Samorukov requested to merge samm-git/gimp:libwmf-path-fix into master

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.

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