WServerGLWidget.C: fix Apple GL for < 10.7

[barracuda156]

Fixes the following error on 10.6:

/opt/local/var/macports/build/wt-4cd339fd/work/wt-4.12.0/src/Wt/WServerGLWidget.C: In constructor 'Wt::WServerGLWidgetImpl::WServerGLWidgetImpl(bool)':
/opt/local/var/macports/build/wt-4cd339fd/work/wt-4.12.0/src/Wt/WServerGLWidget.C:257:5: error: 'kCGLPFAOpenGLProfile' was not declared in this scope
  257 |     kCGLPFAOpenGLProfile, // core profile with the version stated below
      |     ^~~~~~~~~~~~~~~~~~~~
/opt/local/var/macports/build/wt-4cd339fd/work/wt-4.12.0/src/Wt/WServerGLWidget.C:258:31: error: 'kCGLOGLPVersion_Legacy' was not declared in this scope
  258 |     (CGLPixelFormatAttribute) kCGLOGLPVersion_Legacy,
      |                               ^~~~~~~~~~~~~~~~~~~~~~
make[2]: *** [src/CMakeFiles/wt.dir/Wt/WServerGLWidget.C.o] Error 1
make[2]: *** Waiting for unfinished jobs....