Add BINARY_WRAPPERS

[bapt]

Summary

BINARY_WRAPPERS allow to push in front of the PATH wrappers for binaries.
This is useful when 2 tools do provide the same feature, but the configure
scripts do test for some variables which are not in one of the tool.

For example m4 in base is mostly compatible with GNU m4 when called with the -g
arguement. This wrapper allows to use it with build systems that do check for it
calling --version and expecting GNU in the arguments.

[mat]

Should't this be a USES instead of some new code in bsd.port.mk?

Like USES= wrapper:gm4 or something.

[bapt]

I do think it is too short for a USES, I can do it, but in this case it seems simpler like this to me.