Dynamic subdirs in Makefile

[ninsbl]

With sparse checkout of addons in g.extension implemented in OSGeo/grass#2895, not all subdirs of the repository exist at all time.

Runing make in a directory below module level is thus likely to fail. This PR should make sure that make is run in all subdirs that contain a Makefile. And instead of hardcoded directory names, relevant subdirs are identified with the find command line tool.

When g.extension is improved to support also the installation of multiple AddOns (which now should be relatively straight forward on UNIX like systems) then make should be run at the root of the addon repository to install all downloaded addons.

Are find, basename, dirname and xargs available on all relevant UNIX like platforms*?
It seems Mac OS has those (but I have only Fedora and Ubuntu to test).

*On MS Windows addons are not compiled anyway.

[tmszi]

@ninsbl with this change we introduce this issue:

GRASS nc_basic_spm_grass7/PERMANENT:~ > g.extension db.join
WARNING: Extension <db.join> already installed. Re-installing...
Fetching <db.join> from <https://github.com/OSGeo/grass-addons> (be
patient)...
Traceback (most recent call last):
  File "/usr/lib64/grass83/scripts/g.extension", line 2877, in <module>
    sys.exit(main())
  File "/usr/lib64/grass83/scripts/g.extension", line 2857, in main
    install_extension(source=source, url=url, xmlurl=xmlurl, branch=branch)
  File "/usr/lib64/grass83/scripts/g.extension", line 1240, in install_extension
    ) = install_extension_std_platforms(
  File "/usr/lib64/grass83/scripts/g.extension", line 2001, in install_extension_std_platforms
    srcdir = download_source_code(
  File "/usr/lib64/grass83/scripts/g.extension", line 1913, in download_source_code
    directory = download_source_code_official_github(
  File "/usr/lib64/grass83/scripts/g.extension", line 1779, in download_source_code_official_github
    ga = GitAdapter(
  File "/usr/lib64/grass83/scripts/g.extension", line 252, in __init__
    self.addons = self._get_addons_list()
  File "/usr/lib64/grass83/scripts/g.extension", line 388, in _get_addons_list
    addons_dict[file_path.split("/")[3]] = "/".join(
IndexError: list index out of range