Clean up handling of -g and -O flags.

AC_PROG_CC adds -g and -O2 to CFLAGS if you're using GCC; don't bother
adding either of them, or -O, ourselves if we're using GCC.

Add -O for all non-GCC compilers.
Note that clang, for example, is considered "gcc" by autoconf.
We require autoconf 2.61 in configure.in; that suffices.

Don't say "we require 2.50" in various macros.
Update/fix comments.
We no longer use the GCC version, so don't fetch it.
We no longer use the GCC version, so don't fetch it.

Fix some indentation while we're at it.