Makefile: Fix final linking of notmuch binary for OS X.

Apparently the OS X linker can't resolve symbols when linking a
program (notmuch) against a library (libnotmuch) when the library
depends on another library (libgmime) that the program doesn't depend
on directly.

For this case, we need to link the program directly against both
libraries, but we don't want to do this on Linux, where the linker can
do this on its own and the explicit, unneeded link would cause
problems.