Build problems on OS X

[Aaron Ecay]

In the process of updating to the latest sources, I've discovered that notmuch
no longer builds on OS X.  As a reply to this email, I'll be sending 4
patches.  The first two are bugfixes; the third adds the infrastructure to the
Makefiles to build a shared library on OS X, since the Mach-O conventions are
somewhat different than the Linux ones.  Modulo makefile style quibbles, this
patch should be correct.

The fourth patch I am not sure of.  Even after adding the proper command line
flags to build the shared binary on OS X, I get symbol not found errors for
Glib/Gmime symbols.  The shared binary links against the shared lib, which in
turn links against Glib and Gmime, but it appears that at least on OS X the
linker won't follow these second-order links, and the notmuch shared binary
must be linked against Glib/Gmime directly.  This patch fixes the build for
me, but it may not be correct on Linux/other Unices.  I can resubmit this
patch to add the extra linker flags only when building for OS X.  If anyone
knows how to get the OS X linker to behave like the Linux one in this regard,
that will likely be a better solution.

Aaron