Bug 697821 - Disable libtheora assembly on XCode 4.2. r=khuey
The version of llvm-gcc included with Apple's XCode 4.2
toolchain can't compile the inline assembly in the libtheora
library. This is a regression from the 4.1 release.
Check for this version and disable assembly if found.
Also update the check for clang < v2.9 from bug 627981,
which had similar problems, to set the same flag.