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.
CompileFlags: add better selection which aCC version gets -AA +hpxstd98 flags
Merge topic 'hpux-compile' into next

500d83ba CompileFlags: add better selection which aCC version gets -AA +hpxstd98 flags
f347fd12 CompileFlags: Add -AA +hpxstd98 on HP-UX 11.11 with aCC
4ca9b008 CompileFlags: Test for C++ features after selecting flags