configure: fix some autoconf-2.69 update issues (branch 7.8)

[nilason]

This renames configure.in to configure.ac, the previous name has long been deprecated and became obsolete with 2.70.

Also addressed:

• No need to set AC_CONFIG_AUX_DIR, default will do
• Remove double quote on macro 'LOC_FP_TEST'

Addresses remaining issues after merging #1867 as reported with #560.

[nilason]

I updated this PR's title and summary to reflect all changes needed to fix #560.

I take the liberty to merge this straight away to enable this to be tested ASAP on Windows in particular.

[ninsbl]

With unixodbc installed in MSYS2, the build works fine without further modifications. So this looks nice with OSGeo4W-V2.

See:
https://github.com/ninsbl/grass/runs/3631639686?check_suite_focus=true
(there seem to be issues with running tests on UBUNTU, but they are probably unrelated!?)

Here is the temporary branch in my fork with the integrated changes:
https://github.com/ninsbl/grass/commits/releasebranch_7_8_osgeo4w_vs

[nilason]

With unixodbc installed in MSYS2, the build works fine without further modifications. So this looks nice with OSGeo4W-V2.

See:
https://github.com/ninsbl/grass/runs/3631639686?check_suite_focus=true
(there seem to be issues with running tests on UBUNTU, but they are probably unrelated!?)

Here is the temporary branch in my fork with the integrated changes:
https://github.com/ninsbl/grass/commits/releasebranch_7_8_osgeo4w_vs

#1883 addresses this for OSGeo4W-V1 as well.