Updated flextGL.c.template

Removed Duplicate Checks
Added possibility to disable loggin with define FLEXT_NO_LOGGING
Fixed compile errors under GCC
  - No __stdcall on Linux
  - FLEXT_C_EXTENSION to handle cases where the compiler is in
    strict mode and produces errors due to void* to function ptr
    not being legal according to the C standard.
Added FLEXT_C_Extension for OSX code
Some style fixes
Merge ssh://github.com/ginkgo/flextGL