MFH: r385016

Convert lang/gjs and lang/spidermonkey24 to USES=compiler:c++11-lib
to unbreak x11-fm/sushi on FreeBSD 8 and 9. This change causes
these libraries to link to the newer version of libstdc++ bundled
with lang/gcc. This causes rtld to load this version when it is
linking sushi at runtime, which is needed by webkit-gtk3, another
shared library linked into sushi.

PR: 196078, 199434, 199435
Approved by: portmgr (delphij)