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

remove `OLD_CPP` pre-compiler directives: they were not valid c++ and should therefore not be supported.