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.
As a side effect, the Makefiles become a tad simpler. On the way,
update to USE_GCC=4.6+ per pending work of linimon and myself.
Approved by: maintainer