libnotmuch: add linker script to declare only notmuch_* symbols as global.

libnotmuch: add linker script to declare only notmuch_* symbols as global.

This is closely tied to gcc and particularly gnu ld, but I guess the
shared library linking code would need to be adjusted to work on a
non-gnu linker anyay.

I had to make a few not-obviously related changes to the
lib/Makefile.local to make this work: libnotmuch_modules is defined
with := and used in place of $^
(cherry picked from commit 014bf85b1c06ff49be2bde5a26433d2cf376cf70)