CompileFlags: Test for C++ features after selecting flags

Some flags we select may affect availability of C++ features.
CompileFlags: Add -AA +hpxstd98 on HP-UX 11.11 with aCC

These flags are needed to enable support for C++98.  Also teach the
'bootstrap' script to add them if necessary.
Merge topic 'hpux-compile'

be53734d CompileFlags: Add -AA +hpxstd98 on HP-UX 11.11 with aCC
f4b9c6bb CompileFlags: Test for C++ features after selecting flags