I'm working on updating graphics/gphoto2 to the latest beta, and found that
it needs GNU m4. Currently, this means doing something like so:

BUILD_DEPENDS+= m4-1.4:../../devel/m4
M4=             ${PREFIX}/bin/gm4

A few other packages do this sort of thing (I see at least devel/autoconf,
devel/gmp, and x11/gtk--). Following the prior art of USE_GMAKE, does it
make sense to add a USE_GM4 knob to bsd.pkg.mk that does the above work?

- Amitai