PowerPC: Detect AltiVec support on OS X libjpeg-turbo's AltiVec SIMD extensions previously assumed that AltiVec instructions were available on all Power Macs that supported OS X 10.4 "Tiger" (the earliest version of OS X that libjpeg-turbo has ever supported), but Tiger can actually run on PowerPC G3 processors, which lack AltiVec instructions. This commit enables run-time detection of AltiVec instructions on OS X/PowerPC systems if AltiVec instructions are not force-enabled at compile time (using -maltivec). This allows the same build of libjpeg-turbo to support G3, G4, and G5 Power Macs. Closes #609