CFLAGS=-m64 needed for gmp with MacOS

[John Rehwinkel]

Since gmp is built as a 64-bit library, the gmp test in configure  
fails on MacOS with an architecture mismatch with libgmp.dylib.   
Adding CFLAGS=-m64 to the ./configure command fixes this.  I don't  
know if it's worth automating this, but I figured I'd point it out.

– John Rehwinkel