Tell pkg-config when tox core is built vs. nacl

This is required so that applications using tox core do not fail to link when
core is built vs. the nacl library.
Fix regression that disabled shared library builds

My previous attempt that was supposed to disable shared library builds
with nacl had a side effect which basically disabled shared libs for all
configurations.

Eventhough AC_DISABLE_SHARED was used inside an if clause it seemed to
take over in any case.

I could not find a clean way around this, so had to override internal
libtool variables. Will check with the libtool people regarding a
cleaner implementation.
Merge pull request #808 from jin-eld/fix-nacl-2

Tell pkg-config when tox core is built vs. nacl
Merge pull request #809 from jin-eld/fix-shared-lib-builds

Fix regression that disabled shared library builds