Adjust buildscripts and test SpiderMonkey 24 on Mac OS X

[Yves]

I don't own a Mac, so someone else needs to take over this task. You basically have to:

1. Apply the most recent patch in #1886
2. Copy the SpiderMonkey source tarbal to libraries/source/spidermonkey (download: ​https://ftp.mozilla.org/pub/mozilla.org/js/mozjs-24.2.0.tar.bz2)
3. Adjust libraries/osx/build-osx-libs.sh (the changes should be more or less the same as for Linux in libraries/source/spidermonkey/build.sh).
4. Build the library using the build script and build 0 A.D.
5. Check if the 0 A.D. build works or if there are any warnings or errors that are specific to the Mac OS X environment.

The following warnings are currently a know issue that probably also happens on the Mac (ignore them):

mozilla/NullPtr.h|29|warning: "_MSC_VER" is not defined [-Wundef]|.

It would be good to have this done as soon as possible. I would like to commit long enough before the next alpha release to get enough testing. I should be ready with the rest of the SpiderMonkey patch in about a week.