json_writer: improve isfinite support on *nix

json_writer: improve isfinite support on *nix

Based on a patches to CMake by:

Ådne Hovda <ahovda@openit.com>:

    commit 7b1cdb00279908cacabada92f8a53e4986465423

    jsoncpp: Provide 'isfinite' implementation on older AIX and HP-UX

    Newer AIX and HP-UX platforms provide 'isfinite' as a <math.h> macro.
    Older versions do not, so add the definition if it is not provided.

Michael Scott <michael.scott@gbgplc.com>:

    commit 9217b678b305d7df7471ba476a81bf28961fdfa3

    jsoncpp: Provide 'isfinite' impl on more HP-UX versions (#15576)

    Some versions of HP-UX do not define 'isfinite' or 'finite' in math.h
    for Itanium when preprocessing with C++, so we have to add the
    definition ourselves instead to map to the internal version.

Joerg Sonnenberger <joerg@bec.de>:

    commit 75644dafe54c21902f14cfe58cb8338b553b69d8

    jsoncpp: Fix compilation as C99 on Solaris

    In C99 mode, Solaris variants may already define isfinite, so check for
    the existence first.