GLversion in the C loader
'"glad.h" contains a definition of GLVersion instead of a mere declaration.
extern should be used with a single definition in "glad.c".

Proposed patch:

``` diff
diff --git a/glad/loader/gl/c.py b/glad/loader/gl/c.py
index 0b72d99..e5a361b 100644
--- a/glad/loader/gl/c.py
+++ b/glad/loader/gl/c.py
@@ -15,6 +15,8 @@ int gladLoadGL(void) {
 '''

 _OPENGL_HAS_EXT = '''
+struct gladGLversionStruct GLVersion;
+
 static int has_ext(const char *ext) {
     if(GLVersion.major < 3) {
         const char *extensions;
@@ -82,7 +84,7 @@ _OPENGL_HEADER = '''
 #define GLAPI extern
 #endif

-struct {
+extern struct gladGLversionStruct {
     int major;
     int minor;
 } GLVersion;
```
