fix _FILE_OFFSET_BITS redefined warning on Solaris/x86

The issue is that string.h header on Solaris includes somehow
/usr/include/sys/feature_tests.h which tests if _FILE_OFFSET_BITS
is defined and if not, then it defines it to 32 if we're compiling
32 bit code (x86). This is simply wrong since we'd like to have it
defined to 64. The issue is solved by including ghcconfig.h first
which defines _FILE_OFFSET_BITS to 64 and feature_tests.h is later
OK with that.