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.
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.
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.
