If we're building an internal lib do not bother including bsd.incs.mk so we
will not install the headers.
This also "solves" a problem with pkgbase where a libXXX-development package
is created and due to how packages are created we add a dependency to a
libXXX package that doesn't exists.
Reported-by: pizzamig
Signed-off-by: Emmanuel Vadot <manu@freebsd.org>