Correctly set RPATH for private libraries
Fix the --bindir/libdir/datadir args passed to premake: they need to
have = in it for premake to understand it. Getting this right makes the
build system correctly set RPATH so that it's set to $libdir/0ad where
we have the private libs.
This makes it possible to drop the 0ad-rpath.patch and 0ad launcher
shell script that set LD_LIBRARY_PATH and instead just rely on the
RPATH and the datadir being correct.