Log Message:
"BUILD_ENV" was misnamed.  It's actually the environment that is passed
to all internal recursive make processes.  As such, rename it to
"PKGSRC_MAKE_ENV".

XXX Note, some of the usage of this variable in package Makefiles seems
XXX incorrect.  They probably want "MAKE_ENV", which is the environment
XXX passed to the make process when running "make" within ${WRKSRC}.