No C++/C99-style comments; not all C compilers necessarily support them.
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.
With .devel, add -g3 for MIPS C and DEC C.

Lifted from the libpcap configure script, which adds -g even without
.devel, presumably so that programs linked with libpcap can get
debugging symbols.

Those compilers are for dead OSes (IRIX, Tru64 UNIX); we should probably
do whatever voodoo is needed to get Sun^WOracle C, IBM's C compiler for
AIX, and HP C to build optimized with debug symbols.
We no longer use the GCC version, so don't fetch it.

Fix some indentation while we're at it.
Update a comment for the previous commit.
Explain why we set -xansi -signed for MIPS C.