Improve style of games/pmars/Makefile

BUILD_DIRS=            src

is equivalent to:

BUILD_MAKE_FLAGS+=    -C src

"${COMPILER_RPATH_FLAG}${X11BASE}/lib"

is preferred over:

-Wl,-rpath,${BUILDLINK_PREFIX.libX11}/lib

No functional change (on NetBSD).

Pointed out by <joerg>

(kamil)