This was originally added for fixing conflicting definitions between
Android and Qt, but times have changed and now this breaks the build
on iOS:
[...]/OpenGLES.framework/Headers/ES3/gl.h:1006:48: error: unknown type name 'gst_qt_GLsync'
GL_API void GL_APIENTRY glGetSynciv (GLsync sync, GLenum pname, GLsizei bufSize, GLsizei* length, GLint* values) OPENGLES_DEPRECATED(ios(7.0, 12.0), tvos(9.0, 12.0));
^
../ext/qt/gstqtgl.h:49:16: note: expanded from macro 'GLsync'
#define GLsync gst_qt_GLsync
^
6 errors generated.
Instead, we simply avoid defining GLsync ourselves if we're using Qt.