Create BUILD and WORK directory macros

This patch introduces two new macros, BUILD and WORK, to permit
relocation of the build/ and work/ directories.  It also makes the
following smaller changes:

* deps/ is now defined by the DEPS macro.  If unset, deps/ is placed in
  the BUILD directory.

* results/ is moved into WORK.

* Compiler flags which track directories now use $(abspath ...) to
  allow for arbitrary paths.

* GitHub CI paths were adjusted to support these new settings.

* DO_* flags are now used as on/off with ifdef testing, rather than
  checking for `true` values.

* mkmf macros have been removed from the coupled test config.

* The default FMS infra has been changed to FMS2 in all components,
  including the configure.ac outside of .testing.

This work will enable testing of multiple FMS libraries in our CI.

.github/actions/testing-setup/action.yml

@@ -28,7 +28,7 @@ runs:
       run: |
         echo "::group::Compile FMS library"
         cd .testing
-        REPORT_ERROR_LOGS=true make deps/lib/libFMS.a -s -j
+        REPORT_ERROR_LOGS=true make build/deps/lib/libFMS.a -s -j
         echo "::endgroup::"
 
     - name: Compile MOM6 in symmetric memory mode