Replace the use of _GCC_BUILD_DEPENDS (which was never meant to be
used outside of Mk/bsd.gcc.mk) by _GCC_RUNTIME. This is still not
ideal, but at least a documented stopgap, and it avoids the manual
construction of the directory to be added to rpath.

Remove -rpath= from _CFLAGS and _LDFLAGS since CFLAGS and LDFLAGS
already have -Wl,-rpath= (and -rpath= itself is not even accepted
by current versions of GCC).

Approved by: maintainer timeout (6 weeks)
Feature safe: yes