autoconf changes

Hi All,

I discovered recently (when building 1.6.2 on solaris 8 sparc) that the autoconf detection for POSIX Threads wasn't quite adequate for my needs. This is partly due to gcc only warning on invalid flags in some cases (-pthread on solaris 8 sparc) but still exiting cleanly.

When I got looking at things, I found several items that I don't think are quite right. For example, any of the --with- options that take a path argument didn't actually use the value supplied when testing for features in a library. If the user didn't also set a good CPPFLAGS/LDFLAGS (which they should, but that's beside the point), things like the deflateUnbound zlib function might be ignored even when present.

I decided to dig in and modify the configure.ac file fairly heavily. What I've put together in the forthcoming patch set does the following:

1. Handles user arguments early in the configure script and makes the
   values that get set in config.mak.autogen available in the rest of
   the script (I find those names nicer that $with_...).  These
   settings now make it into config.log too.
2. Use the values supplied to wrap tests with a modified CPP/LD FLAGS
   pair when applicable.
3. Fixed the asciidoc test to better handle python 2.6 (I haven't
   tested with 2.4, but the change is quite simple and shouldn't be a
   problem [as someone else mentioned recently: famous last words]).
4. Allows the user to pass in compiler flags to enable POSIX Threads
   which will override the default pair of tests.
5. Tries to maintain the original actions and intents where possible.

I've tested this on solaris 8 and Linux (Ubuntu 9.04) and it seems to do all the things I'd expect.

I hope you'll find these to be useful changes.

Thanks -Ben