package/exim: fix static linking

Fixes http://autobuild.buildroot.net/results/0a3/0a392087878f80a70435981856455a30152b684d/



When building static, we need to tell exim that it needs to link
statically (obviously), and that it should link against pthreads.

Yet, exim is still not happy with that, as it still wants to build
something (a version-related program) at install time, so we need
to replicate the ld flags at install time.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>