Add signed integer overflow checking to flac.

Migrate flac over to Clang in order to use its -ftrapv option to abort
on signed overflow.

Change-Id: I9f92459b6b5ecfdc49377a191db8d0265f190ab1
Merge tag 'tags/android-4.4_r1' into kitkat-staging

Android 4.4 Release 1.0