Speeding up pkg_version and perhaps other port utilities

[Stephen Montgomery-Smith]

On my system, the program pkg_version can double its speed simply by 
replacing "make -V PKGNAME" by "make BEFOREPORTMK=yes -V PKGNAME" in 
src/usr.sbin/pkg_install/version.  Basically what it does is to tell 
"make" to process only about half of bsd.port.mk.

It seems to me that with some clever use like this, certain commands 
like "make all-package-depends" could selectively read only parts of 
bsd.port.mk.