Enable GL prototypes with recent Khronos headers

Recent Khronos headers decided to break the world by guarding all
function prototypes with GL_GLEXT_PROTOTYPES which has traditionally
been used for extension headers only.

Until this gets corrected - see
https://lists.freedesktop.org/archives/mesa-dev/2016-September/128654.html
- add the define to the config tests and qopengl.h.

While 5.7 already has some of the qopengl.h fixes due to an upgraded
ANGLE shipping with newer headers, this is a cross-platform issue that
will surface everywhere eventually. Therefore we target the full set
of fixes to 5.6.

This time we also make sure the forced define of GL_GLEXT_PROTOTYPES
is removed before including the ext header, thus apps get the ext
protos only if they actually requested them.

Task-number: QTBUG-56764
Change-Id: Ib2c6d2e7b71b8fb8683424f43e6289e64e4ee46c
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

config.tests/unix/opengldesktop/opengldesktop.cpp

@@ -34,6 +34,7 @@
 #ifdef __APPLE__
 #include <OpenGL/gl.h>
 #else
+#define GL_GLEXT_PROTOTYPES
 #include <GL/gl.h>
 #endif
 

config.tests/unix/opengles2/opengles2.cpp

@@ -34,6 +34,7 @@
 #ifdef BUILD_ON_MAC
   #include <OpenGLES/ES2/gl.h>
 #else
+  #define GL_GLEXT_PROTOTYPES
   #include <GLES2/gl2.h>
 #endif
 

config.tests/unix/opengles3/opengles3.cpp

@@ -34,6 +34,7 @@
 #ifdef BUILD_ON_MAC
   #include <OpenGLES/ES3/gl.h>
 #else
+  #define GL_GLEXT_PROTOTYPES
   #include <GLES3/gl3.h>
 #endif
 

src/gui/opengl/qopengl.h

@@ -89,6 +89,13 @@ typedef void* GLeglImageOES;
 // applications cannot target ES 3. Therefore QOpenGLFunctions and
 // friends do everything dynamically and never rely on these macros.
 
+// Some Khronos headers use the ext proto guard in the standard headers as well,
+// which is bad. Work it around, but avoid spilling over to the ext header.
+#  ifndef GL_GLEXT_PROTOTYPES
+#   define GL_GLEXT_PROTOTYPES
+#   define QGL_TEMP_GLEXT_PROTO
+#  endif
+
 #  if defined(QT_OPENGL_ES_3_1)
 #   include <GLES3/gl31.h>
 #  elif defined(QT_OPENGL_ES_3)
@@ -97,6 +104,11 @@ typedef void* GLeglImageOES;
 #   include <GLES2/gl2.h>
 #endif
 
+#  ifdef QGL_TEMP_GLEXT_PROTO
+#   undef GL_GLEXT_PROTOTYPES
+#   undef QGL_TEMP_GLEXT_PROTO
+# endif
+
 /*
    Some GLES2 implementations (like the one on Harmattan) are missing the
    typedef for GLchar. Work around it here by adding it. The Kkronos headers
@@ -117,7 +129,15 @@ typedef char GLchar;
 #  include <OpenGL/glext.h>
 # else
 #  define GL_GLEXT_LEGACY // Prevents GL/gl.h from #including system glext.h
-#  include <GL/gl.h>
+// Some Khronos headers use the ext proto guard in the standard headers as well,
+// which is bad. Work it around, but avoid spilling over to the ext header.
+#  ifndef GL_GLEXT_PROTOTYPES
+#   define GL_GLEXT_PROTOTYPES
+#   include <GL/gl.h>
+#   undef GL_GLEXT_PROTOTYPES
+#  else
+#   include <GL/gl.h>
+#  endif
 #  include <QtGui/qopenglext.h>
 # endif // Q_OS_MAC
 #endif // QT_OPENGL_ES_2