package's prefix would not work as part of the environment specification
via MAKE_ENV (as it would not be executed in the correct directory).
Fix this by invoking pkg_info(1) directly, not via an intermediate make(1)
step - this is not as clean, but more effective (i.e. it works).
a bit more user-friendly.
Introduce a show-{gtk+,imlib,kdebase,qt1,qt2,xpm}-prefix target in
bsd.pkg.mk, and use "${MAKE} show-*-prefix" in package Makefiles.
