Add workaround for defective <cmath>

For older compilers (pre-C++11) picojson accesses the isnan and isinf
functions by including <math.h>. But this can lead to problems if the
user included <cmath> before including "picojson.h": some
implementations of <cmath> include <math.h> themselves and then
undefine some macros like isnan and isinf as not needed before C++11.
This happens at least for:
g++ (rev2, Built by MinGW-builds project) 4.8.0

This workaround relies on the fact that if the <math.h> include inside
"picojsoon.h" fails because of a previous <cmath> include, then
std::isnan and std::isinf are available. And if in contrast <math.h> is
properly included, isnan and isinf are implemented as preprocessor
macros.
Merge pull request #38 from olafmandel/issue_38

Add workaround for defective <cmath>