Build slaves on automation are based on Centos 6, which doesn't have a
recent enough version of libstdc++ for our new requirements. But since
we're building with a recent GCC or clang with its own libstdc++, we do
have such a libstdc++ available somewhere, and the compiler picks it
when invoking the linker.
Problems start happening when we execute some of the built programs
during the build, like host tools (e.g. nsinstall), or target programs
(xpcshell, during packaging). In that case, we need the compiler's
libstdc++ to be used. Which required adding the GCC or clang library
directory to LD_LIBRARY_PATH.
Unconveniently enough, the clang 3.5 tooltool package we're using for
ASAN builds until we can update to at least 3.8 (bug 1278718) doesn't
contain libstdc++.so. So for those builds, pull the GCC package from
tooltool as well, and pick libstdc++ from there.
Some mozconfigs don't include mozconfig.linux*, and don't get gtk-related
definitions, so move them in a separate mozconfig. To avoid having two
files, one for 32-bit builds and one for 64-bit builds, rely on the
includer to set PKG_CONFIG_LIBDIR appropriately.
At the same time, move all the --enable-default-toolkit=cairo-gtk2 in that
new file in the case the gtk3 package wasn't pulled from tooltool.
