enc_neon: fix building with non-Xcode clang (iOS)

check for __apple_build_version__ to distinguish the two; a version
check could work as Apple bumped Xcode's to 5.x/6.x, but it's unclear
how upstream will deal with their versioning as they go 3.6+, so avoid
it for now.

Change-Id: I67cda67c4f68e262a92d805a63cc1496374be063
Merge "enc_neon: fix building with non-Xcode clang (iOS)"