patch to have configure check if CC is intel C compiler

If configure sees that the compiler specified by $CC looks like gcc
(defines __GNUC__), then it puts some extra command line options into the
CFLAGS (mostly -W*). The intel C compiler for linux emulates gcc by
default, which means it defines that and looks very much like gcc to
configure. However, it does not get along with the added -W flags very
well. They don't seem to kill it, but some of them give warnings about
unsupported command line options and others produce insane amounts of
output from the compiler.

This patch makes configure check for the __INTEL_COMPILER define (which is
the recommended way to detect the intel compiler) before adding the extra
CFLAGS if it thinks the compiler is GCC. I am not an autoconf hacker, so
I may have done it wrong or something, but it appears to work for me (ICC
9.0.032 on gentoo i686 with latest packages).

The patch is against the HEAD but I think it should be similar for other
branches (I have been compiling 8.0 and 8.1 for some time with the intel
compiler by manually massaging the makefiles after configure ran).

--
We the unwilling, led by the ungrateful, are doing the impossible.
We've done so much, for so long, with so little,
that we are now qualified to do something with nothing.