JS_STANDALONE builds don't need everything in mozglue; have mozglue omit the
unneeded code for such builds.
Since the SpiderMonkey binaries are now more like the other Gecko binaries, we
can remove some special cases for JS_STANDALONE in GeckoSharedLibrary,
GeckoProgram, etc. All Gecko binaries now use mozglue, which contains mfbt, so
we no longer need mention mfbt explicitly.
Add a property to the getBuildConfiguration testing function's result that
indicates whether we're using jemalloc or not.
Include the newly necessary source directories in the SpiderMonkey source
package.
