Fix #29 C++ Style Includes

**Fix #29 C++ Style Includes**

- c std libs should be included <cLIBNAME> in C++
  http://www.cplusplus.com/reference/clibrary/
- gcc 4.8+ failed because pngwriter uses cstring and not c++ string which are different libraries (but probably pulled some indirect includes before)
- link `libm` explicitly, too