m4/ax_tls_support.m4: Check that thread-local pointers to classes are usable.

configure: Regenerate.
configure.ac: Provide -Bhidden_def to aCC when __declspec(dllexport)
is detected. Tweak logic around use_export_symbols_regex.

configure: Regenerate.
README: Add note about HP-UX with aCC.
README: Add HP-UX with aCC as a tested platform.
stringhelper.h: Apease HP aCC compiler.
env.cxx: Appease IBM xlC.
src/Makefile.am: Avoid GCC specific symbols matching regex.