Hook HAS_TESTS (r321901) in to bsd.lib.mk and bsd.prog.mk

Apply the required sugar to make "make check" just work (tm) when
invoked from a subdirectory where HAS_TESTS is defined in the
relevant Makefile.

Adjust $LD_LIBRARY_PATH and $PATH appropriately from bsd.lib.mk
and bsd.prog.mk to make the "user experience" more out of the
box/seamless.