Enable the __builtin_ia32_crc* acceleration functions for gcc-4.7

Summary:
Adjusted the ifdefs to allow the use of the hardware CRC instructions
when compiling with gcc-4.7.x

Test Plan:
Built with gcc-4.7.1, ran the unit tests, verified that they ran the
hardware-accelerated code path

Reviewed By: saramg@fb.com

FB internal diff: D1134567

folly/Checksum.cpp

@@ -25,7 +25,7 @@ namespace folly {
 namespace detail {
 
 #if defined(__x86_64__) && defined(__GNUC__) && defined(__GNUC_MINOR__) && \
-    (((__GNUC__ * 100) + __GNUC_MINOR__) >= 408)
+    (((__GNUC__ * 100) + __GNUC_MINOR__) >= 407)
 
 // Fast SIMD implementation of CRC-32C for x86 with SSE 4.2
 uint32_t crc32c_hw(const uint8_t *data, size_t nbytes,