Makefile: Speedup sphinx-apidoc execution significantly
Previously the file patterns that sphinx-apidoc must exclude, stored
in the APIDOC_EXCLUDES variables, were expanded by the shell before
executing the tool resulting in a really long list of files to exclude.
Each time a file was processed by sphinx-apidoc, its path was compared
against that long list of files which was significantly slowing down
the apidoc generation process.
So ensure file exclude patterns are quoted to avoid shell expansion
and greatly speedup sphinx-apidoc execution.