Building against libstdc++ effectively always requires -pthread anyway (as
various standard C++ headers require it, see the comment added to
solenv/gbuild/platform/unxgcc.mk), so many explicit uses of -pthread/-lpthread
can be removed.

Doing a (partial) test build on Linux with Clang -stdlib=libc++ suggests that
libc++ indeed doesn't need -pthread as libstdc++ does.

The remaining uses of -pthread/-lpthread are mostly in configure.ac for the
various BSDs (which somebody else might want to clean up now), and related to
external projects.  I tried to be careful to remove -pthread/-lpthread from
makefiles only when C++ object files are involved (so -pthread will now be
included on the link command line by default).

Change-Id: I936e082839cb9a434bd273ce5a1f187a4245dfa1
Reviewed-on: https://gerrit.libreoffice.org/71291
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>