compiler specific cflags

[Sean MacLennan]

This is probably in a FAQ somewhere, if someone knows where, a pointer
would be appreciated!

I want to set the -Wall flags iff the compiler supports it (i.e. is
gcc). How do I do this? Specifically checking for gcc is fine.

As a secondary concern, I also want to set the flags based on a given
architecture. I guess I really want to know how automake chooses the
default cflags.

Cheers,
   Sean MacLennan