Since proto-standard is used to build a library that can provide
defaults for the extension, include its Makefile regardless.  This has
no technical effect at all until we remove duplicated code (next
commit).

This commit also makes some trivial and small changes in variable
assignements (the ones the user may want to change are now at the top
of the file).
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>