Don't hardcode an include path to the xcode default toolchain

The necessary system include paths are already inherited, and manually
forcing this location can cause conflicts if using different versions
of xcode installed in parallel.

None of the other xcode projects hardcode this path.
Don't hardcode the sdk on the target level

It's already set on the project level, and this is the default
value anyway.

This was the only project where the sdk was set on the target level.
Updated to match gmp-api changes for Firefox33
Changed gmp-api branch to master and astyled the changes.
Merge pull request #1155 from ethanhugg/gmpapi-ff33

Updated to match gmp-api changes for Firefox33
Merge pull request #1147 from mstorsjo/avoid-hardcoded-xcode-paths

Don't hardcode an include path to the xcode default toolchain
Merge pull request #1150 from mstorsjo/unify-sdk-setting

Don't hardcode the sdk on the target level
Merge remote-tracking branch 'upstream/master' into travis.osx