Add fallback implementation for extension functions.

The new statically linked library libfallback.a provides empty
implementations for get_custom_options(), known_fields_expected() and
find_known_field().  If no other source file defines these functions,
the empty ones will be used from the library.
Ignore libfallback.a and ar-lib in git.
Split options and known_fields handling in libfallback.

Two separate object files allow to pull only part of the functionality
from libfallback.a if one of the dummy_* objects is already linked in.
Rename dummy_* modules to custom_*.
Hide custom_* modules behind configure switches.

Add configure script options --enable-custom-options and
--enable-custom-fields for triggering automake conditional building of
the respective source files.
Adapt Makefile.simple to new custom_* modules.
Merge branch 'custom-extensions' into develop

Conflicts:
	src/Makefile.simple