test: Fix from/to search test queries

Two of the search tests for "from" and "to" queries were clearly
trying to search for prefixed phrases, but forgot to shell quote the
phrases.  Fix this by quoting them correctly.
test: Add search tests for combined name/address queries

Two of these are currently known-broken.  We index the name and
address parts in two separate calls to _notmuch_message_gen_terms.
Currently this has the effect of placing the term positions of the
prefixed terms from the second call right after those of the first
call, but screws up the term positions of the non-prefixed terms.
lib: Index name and address of from/to headers as a phrase

Previously, we indexed the name and address parts of from/to headers
with two calls to _notmuch_message_gen_terms.  In general, this
indicates that these parts are separate phrases.  However, because of
an implementation quirk, the two calls to _notmuch_message_gen_terms
generated adjacent term positions for the prefixed terms, which
happens to be the right thing to do in this case, but the wrong thing
to do for all other calls.  Furthermore, _notmuch_message_gen_terms
produced potentially overlapping term positions for the un-prefixed
copies of the terms, which is simply wrong.

This change indexes both the name and address in a single call to
_notmuch_message_gen_terms, indicating that they should be part of a
single phrase.  This masks the problem with the un-prefixed terms
(fixing the two known-broken tests) and puts us in a position to fix
the unintentionally phrases generated by other calls to
_notmuch_message_gen_terms.
test: Known-broken test for overlapping/adjacent termpos

This adds two known-broken tests and one working test related to the
term positions assigned to terms from different headers or MIME parts.
The first test fails because we don't create a termpos gap between
different headers.  The second test fails because we don't adjust
termpos at all when indexing multiple parts.
lib: Separate all phrases indexed by _notmuch_message_gen_terms

This adds a 100 termpos gap between all phrases indexed by
_notmuch_message_gen_terms.  This fixes a bug where terms from the end
of one header and the beginning of another header could match together
in a single phrase and a separate bug where term positions of
un-prefixed terms overlapped.

This fix only affects newly indexed messages.  Messages that are
already indexed won't benefit from this fix without re-indexing, but
the fix won't make things any worse for existing messages.
configure: add workaround for systems without zlib.pc

Some systems (e.g. FreeBSD) might not have installed the appropriate
pkg-config file as they should. We can workaround the issue by creating
the .pc file they should have distributed.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>