Recent linker errors for notmuch-shared

[Ben Gamari]

Hey all,

When I pulled a few days ago I noticed that notmuch-shared failed to link due
to missing LDFLAGS. In particular, it seems that the linker flags don't include
CONFIGURE_LDFLAGS, meaning that all of the required external shared libraries
depedencies aren't linked against. Here is a patch to fix this. It works for
me, although perhaps this isn't what others envision. Thanks!

- Ben