Fix wrong configure test for abi::__forced_unwind in previous commit.

Fix wrong configure test for abi::__forced_unwind in previous commit.

The previous commit was accidental and contained an initial version of the
patch which didn't test for NPTL abi::__forced_unwind correctly and just
tested whether cxxabi.h header was available.

Tighten the check to work on the other systems and check for __forced_unwind
existence itself.

Also check for cxxabi.h before testing for __cxa_demangle as there is no need
to try to compile another test program if we already know that this entire
header is unavailable anyhow.

See #14626.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72532 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775