  - find why std::isnan required

    gdt: As I understand it, standards permit but do not require
    implementations to make names defined in the std namespace
    available globally.  It seems to be linuxy to assume they are
    available.  The use of std::isnan is proper, and I just had a
    s/isnan/std::isnan/ change merged to qgis today.  Really these
    should just all be fixed upstream.
    (Perhaps they could be SUBST.)

  - check other system library use
