Unlike other "add doxygen" commits this one adds a preprocessor branch
that doxygen would process.  This prevents it from parsing other parts
of the file.

Truly editorial fix for comments.  This works better with Emacs'
set-justification-full function. [ci skip]

Reported by Mau Magnaguagno See: https://github.com/ruby/ruby/pull/3570#discussion_r495465903

Since r63443, `-std=gnu99 -D_XOPEN_SOUCE=x00` is added to Solaris'
`CPPFLAGS`.  `CPPFLAGS` is shared among `CC` / `CXX`.  This results in
both `__STDC_VERSION__` and `__cplusplus` to be defined at the same time
for a C++ compilation, only on Solaris.

It seems the `CPPFLAGS` addition is intentional.  We sould not touch
that part.  Instead we need to reroute this by always check for
`__cplusplus` first.

Added description and URL about nested flexible array member.

Was definitely a bad idea to use constexpr.  It is not ubiquitous.

It is reported that on a system of i386 System V ABI, GCC returns 8 for
__alignof__(double).  OTOH the ABI defines alignments of double to be 4,
and ISO/IEC 9899:2011 reads that _Alignof(double) shall return 4 on such
machine.  What we want in ruby is 4 instead of 8 there.  We cannot use
__alignof__.

Additionally, both old GCC / old clang return 8 for _Alignof(double) on
such platforms.  They are their bugs, and already fixed in recent
versions.  But we have to support older compilers for a while.  Shall
check sanity of _Alignof.

To fix build failures.

Devs do not love "impl".