lib: make notmuch shared library install_name be full path on Mac OS X

lib: make notmuch shared library install_name be full path on Mac OS X

The install_name of libnotmuch.dylib on Mac OS X is what is written
into a program that links against it.  If it is just the name of the
shared library file, as opposed to the full path, the program won't be
able to find it when it runs and will abort.  Instead, the install_name
should be the full path to the shared library (in its final installed
location).

Why does Notmuch work without this patch when installed via Homebrew?
The answer is twofold.  One, /usr/local/lib is a special location in
which the dynamic linker will look by default to find shared libraries.
Homebrew highly recommends installing to /usr/local, and, assuming it
has been configured this way, the Notmuch library will end up installed
in /usr/local/lib, and the dynamic linker will find it.  Two, Homebrew
globally corrects all install names in dynamically shared libraries and
binaries for each package it installs.  So, even if the install names in
a package's binaries and libraries are incorrect, Homebrew corrects them
automatically, and no one ever knows.

Why does Notmuch work without this patch when installed via MacPorts?
The answer is that MacPorts applies a patch just like this patch to fix
the same problem.