* meson: fix regression that broke extracting version
In commit 33f67386fe the code that
heuristically parsed the version broke due to the version being moved
around into a more easily accessible define.
While we are at it, pass the exact path of httplib.h to un-break usage
as a meson subproject. This was broken in commit
8ecdb11979 which checked the return code
of trying to get the version; it was always broken, but formerly failed
in silence and resulted in no version number.
* meson: use the compiler builtins to extract the version from the header
As a convenient string define, it is now possible to ask the
preprocessor what the version of cpp-httplib is. This can be used from
meson too, in order to avoid encoding C++ file structure into python
regexes.
