When ENABLE_RELOCATABLE_RESOURCES is set, override libwmf fontdir. This
is actually an alternate version of MR !9 by Alex Samorukov assuming a
bundled GIMP on a single prefix rather than depending on an environment
variable.
This especially makes the relocatable feature more discoverable (rather
than some random environment variable for which you'd need to read the
code to discover it then make some wrapper script for GIMP).

(cherry picked from commit 4ff856f6)