configure: calculate NOTMUCH_BUILDDIR, write to Makefile.config

This will correct the current use of an undefined variable when
setting LD_LIBRARY_PATH in doc/Makefile.local

It is tempting to try to replace the use of test/export-dirs.sh, but
this is not as simple as it looks, as NOTMUCH_BUILDDIR is used to
locate sh.config, so probably cannot also sensibly be used to define
it.