Pre-built configure script

[marshallward]

This PR makes several changes to normalize the autoconf build:

• A prebuilt ./configure script has been added to the top-level.

  This script is now more general-purpose, and can be shared across all MOM6 builds, including .testing/ and parent repositories like MOM6-examples/. No more need to copy configure.ac and m4/ into build directories and calling autoconf.

  ./configure contains checks to prevent build in both the top directory and .testing/. This is primarily to prevent clobbering of the existing Makefiles.

• Much of the ac/ "strangeness" of the older layout has been fixed: We still keep all supporting autoconf content in ac/ but no longer need to do elaborate directory-hopping to normalize our working directory.

• Makefile.in was still using $SRC_DIRS to test Makefile.dep rebuilds, even though this has been replaced with MAKEDEP_FLAGS. To fix this, source directories have been separated from MAKEDEP_FLAGS and moved to MAKEDEP_DIRS. (SRC_DIRS was also replaced.)

• A top-level Makefile has been added to execute ./configure and execute ac/deps/Makefile if necessary.

• The mom6_fc_check_lib.m4 macro now uses m4_normalize to correctly display more exotic paths. The legacy ax_fc_check_lib.m4 has also been removed.

• .gitignore cleanups: More targeted output, removal of vague generic targets.

• A missing siglongjmp was incorrectly set to SETJMP. This is now fixed.

[marshallward]

I was unable to achieve my true goal of completely eliminating autoconf/automake as a dependency. Although configure is pre-built and fully featured, we still use autoconf in a few other places.

• tc4/configure inputs
• ac/deps/{fms/cvmix/gsw}

These could be solved in a few ways:

• add four more configure scripts (at >100kB each)
• use pre-built tc4/ inputs
• drop the independent CVMix and GSW builds

but I wasn't happy with any of those solutions and it was preventing submission of this work, which is completed and in the right direction.

[marshallward]

I'm having second thoughts about some of the decisions here. It seems more common for a repository codebase to provide a "bootstrap" or "autogen" script to generate ./configure. So I will probably make an amendment to this which removes the prebuilt ./configure, but keep the structural changes.

[marshallward]

I updated this to remove the prebuilt configure, but for all MOM6 build to use the (generated) top-configure. I think that is the right design here.

BTW any project assuming the old format will probably need to be modified (e.g. MOM6-examples Makefile builds). Probably won't affect existing regression testing though.