Parsing issue with GL
Seeing the following error when trying to use pretty much any variant of desktop GL. Using the Python module directly results in a segfault but CMake output more details.

Python version: `3.7`
Arguments: `--api gl:core=4.5`

```
  Generating external/glad_gl_45_core/include/KHR/khrplatform.h, external/glad_gl_45_core/include/glad/gl.h, external/glad_gl_45_core/src/gl.c, external/glad_gl_45_core/args.txt
  [09.09.2018 10:30:47][DEBUG   ][glad.plugin   ]: loaded language c: <class 'glad.generator.c.__init__.CGenerator'>
  [09.09.2018 10:30:47][DEBUG   ][glad.plugin   ]: loaded language rust: <class 'glad.generator.rust.__init__.RustGenerator'>
  [09.09.2018 10:30:47][DEBUG   ][glad.plugin   ]: loaded specification egl: <class 'glad.specification.EGL'>
  [09.09.2018 10:30:47][DEBUG   ][glad.plugin   ]: loaded specification gl: <class 'glad.specification.GL'>
  [09.09.2018 10:30:47][DEBUG   ][glad.plugin   ]: loaded specification glx: <class 'glad.specification.GLX'>
  [09.09.2018 10:30:47][DEBUG   ][glad.plugin   ]: loaded specification vk: <class 'glad.specification.VK'>
  [09.09.2018 10:30:47][DEBUG   ][glad.plugin   ]: loaded specification wgl: <class 'glad.specification.WGL'>
  [09.09.2018 10:30:47][INFO    ][glad          ]: getting 'gl' specification from remote location
  [09.09.2018 10:30:47][INFO    ][glad.opener   ]: opening: 'https://raw.githubusercontent.com/KhronosGroup/OpenGL-Registry/master/xml/gl.xml'
  [09.09.2018 10:30:48][INFO    ][glad          ]: generating gl:core/gl=Version(major=4, minor=5)
  [09.09.2018 10:30:48][INFO    ][glad          ]: adding all extensions for api gl to result
  Traceback (most recent call last):
    File "C:\Users\dave\Development\glad_test\lib\glad\glad\parse.py", line 499, in find
      raise StopIteration
  StopIteration

  The above exception was the direct cause of the following exception:

  Traceback (most recent call last):
    File "C:\Users\dave\AppData\Local\Programs\Python\Python37\lib\runpy.py", line 193, in _run_module_as_main
      "__main__", mod_spec)
    File "C:\Users\dave\AppData\Local\Programs\Python\Python37\lib\runpy.py", line 85, in _run_code
      exec(code, run_globals)
    File "C:\Users\dave\Development\glad_test\lib\glad\glad\__main__.py", line 177, in <module>
      main()
    File "C:\Users\dave\Development\glad_test\lib\glad\glad\__main__.py", line 164, in main
      feature_sets = list(select(specification, api, info) for api, info in apis)
    File "C:\Users\dave\Development\glad_test\lib\glad\glad\__main__.py", line 164, in <genexpr>
      feature_sets = list(select(specification, api, info) for api, info in apis)
    File "C:\Users\dave\Development\glad_test\lib\glad\glad\__main__.py", line 161, in select
      return generator.select(specification, api, info.version, info.profile, extensions, config, sink=logging_sink)
    File "C:\Users\dave\Development\glad_test\lib\glad\glad\generator\c\__init__.py", line 239, in select
      return JinjaGenerator.select(self, spec, api, version, profile, extensions, config, sink=sink)
    File "C:\Users\dave\Development\glad_test\lib\glad\glad\generator\__init__.py", line 60, in select
      return spec.select(api, version, profile, extensions, sink=sink)
    File "C:\Users\dave\Development\glad_test\lib\glad\glad\parse.py", line 651, in select
      result = result.union(found)
  RuntimeError: generator raised StopIteration
```