BuildSystem: initial Xcode5 support

Tested on MacOSX 10.8.5 w/ Xcode 5.0, 4.6.3 and 4.5.2, various builds,
including terminal Xcode, terminal no Xcode and Xcode IDE,
and permutations with/without mp4v2 and mkv.

- push defs for optional static libraries (OSL) to respective modules
- adjust make-driven linking to use OSL
- adjust Xcode-driven linking to use OSL via ld -filelist option
- add Xcode project group for all OSL
- set Xcode project to use clang compiler
- add osx 10.8 xcconfig files
- add "supplemental" xcconfig file where EXTERNAL_CONFIGURE may be used
  to specify configure-time options
- patch mp4v2 to compile with clang

Intructions for custom configure-options within Xcode IDE:

1. edit macosx/xcconfig/supplemental.xcconfig
2. save file (unsure how long it takes IDE to see change but it does)
3. build clean
4. build
5. never commit supplemental.xcconfig to repository - it is meant to
   exist but not effect the build on a clean checkout, and to be used
   only as a developer transient developer customization file.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5800 b64f7644-9d1e-0410-96f1-a4d463321fa5