graphics/gimp-beautify-plugin: fix build with GCC-based architectures

[pkubaj]

Summary

Respect CFLAGS and LDFLAGS to fix build when using GCC.

Also add USES=gnome.

[mat]

graphics/gimp-beautify-plugin/Makefile

40–42 ↗

(On Diff #57125)

This could probably be written as:

PLIST_FILES=   ${PLUGINS:S,^,${PLUGIN_DIR}/,}

(but I am nitpicking.)