Fix build in Harmattan SDK (closes #32)

Thanks to coderus for noticing. The following changes were necessary:

 - KHR/khrplatform.h - This includes some typedefs that are needed
 - GLES2/gl2platform.h - Also some definitions from here are needed

Remove GL_APICALL and GL_APIENTRY - they are not used when defining
function pointers, and we don't have different calling conventions.