configure: Bump minimum-supported gcc version to 4.7

Since the __atomic_* builtins are not supported until gcc 4.7. Given
that this version was released in 2012 I think this is acceptable.

configure.ac

@@ -765,7 +765,7 @@ AC_SYS_INTERPRETER()
 
 dnl ** look for GCC and find out which version
 dnl     Figure out which C compiler to use.  Gcc is preferred.
-dnl     If gcc, make sure it's at least 4.6
+dnl     If gcc, make sure it's at least 4.7
 dnl
 FP_GCC_VERSION