More autoconf housekeeping

Drop the test for getopt, it's not used anywhere anymore.

Switch the last uses of AC_TRY_COMPILE to AC_COMPILE_IFELSE now.
The former is marked as obsolete, and this will leave no confusion
about which to cut and paste if new tests are added.

Double quote all the parameters to AC_LANG_SOURCE and AC_LANG_PROGRAM.
This is actually required, even if you can get away with not doing it
sometimes, so again set a good example for future changes to follow,
to hopefully avoid people getting bitten harder than they need to be.

Don't bother checking for alloca if we're never going to use it
(ie. if we have C99 variable-size array support).
The test for this is a bit sketchy anyway ...  we separately test for
HAVE_ALLOCA_H and USE_ALLOCA, but the test for USE_ALLOCA depends upon
having alloca.h present, yet the use of these macros in stack_alloc.h
only tests for HAVE_ALLOCA_H inside of a test for USE_ALLOCA.
I'm not going to change this logic right now, since I don't know what
crazy system it was attempting to cater for, though I suspect it was
one that was not using the autoconf build system ... since with the
current test that combination should not be possible to obtain.

Use LT_LIB_M instead of the song and dance with testing for exp().
This should also work for BeOS which is what the exp test was added for.
It also means we don't unconditionally add -lm to everything via LIBS.

Use LIBM now instead of hardcoding -lm everywhere.

Use AS_HELP_STRING to format all option descriptions.

Don't bother to test for doxygen if using it is --disable'd.

Drop the SYMBOL_VISIBILITY export, it isn't used anywhere (we add the
compiler flag to CFLAGS).