build: move LDFLAGS after notmuch libraries.

build: move LDFLAGS after notmuch libraries.

In [1] Ryan Schmidt reported a problem on macports [2] with notmuch
finding an existing installed version of libnotmuch during the build
when the user specified LDFLAGS including the libnotmuch install
directory.

This change should prevent that.

LDFLAGS also occurs in FINAL_LIBNOTMUCH_LDFLAGS. The only built
library linked to that is util/libtnotmuch_util.a, and that passed as
explicit (relative) path, and is thus not affected by -L.

[1]: id:7851CAB5-4556-4931-A0A2-37003E56C927@ryandesign.com

[2]: The problem does not arise when libnotmuch is installed into a
"system" library path that the compiler/linker searches by default.