build: fix order of rpath

In my system `pkg-config --libs talloc` returns
'Wl,-rpath,/usr/lib -ltalloc' (probably wrongly) which causes the final
LDFLAGS to be something like '-Wl,-rpath,/usr/lib
-Wl,-rpath,/opt/notmuch/lib', which causes the RUNPATH to be
'/usr/lib:/opt/notmuch/lib', so basically defeating the whole purpose of
RUNPATH.

I noticed this when my /opt/notmuch/bin/notmuch (0.17) started updating
the database after I updated the system (which updated the system's
notmuch). This shouldn't happen.

Let's move the RUNPATH flags before other external flags have a chance of
screwing the build.

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