gl/meson: fix macos additions for non-macos platforms

"gst-libs/gst/gl/meson.build:655:20: ERROR:  Unknown variable "quartzcore_dep"."

gst-libs/gst/gl/meson.build

@@ -624,6 +624,17 @@ if need_platform_wgl != 'no' and need_win_win32 != 'no'
   endif
 endif
 
+# OSX check
+if need_platform_cgl == 'yes'
+  if need_win_cocoa == 'no'
+    error('Impossible situation requested: Cannot use CGL without Cocoa support')
+  elif need_api_opengl == 'no'
+    error('Impossible situation requested: Cannot use CGL without the OpenGL library')
+  endif
+elif need_platform_cgl == 'no' and need_win_cocoa == 'yes'
+  error('Impossible situation requested: Cannot use Cocoa without CGL support')
+endif
+
 if host_machine.system() == 'darwin'
   if not have_objc
     error('No ObjC compiler found')
@@ -638,20 +649,8 @@ if host_machine.system() == 'darwin'
 
   quartzcore_dep = dependency('QuartzCore', required : false)
   corefoundation_dep = dependency('CoreFoundation', required : false)
-endif
 
-# OSX check
+  if need_platform_cgl != 'no'
-if need_platform_cgl == 'yes'
-  if need_win_cocoa == 'no'
-    error('Impossible situation requested: Cannot use CGL without Cocoa support')
-  elif need_api_opengl == 'no'
-    error('Impossible situation requested: Cannot use CGL without the OpenGL library')
-  endif
-elif need_platform_cgl == 'no' and need_win_cocoa == 'yes'
-  error('Impossible situation requested: Cannot use Cocoa without CGL support')
-endif
-
-if need_platform_cgl != 'no'
     if quartzcore_dep.found() and corefoundation_dep.found()
       gl_platform_deps += [quartzcore_dep, corefoundation_dep]
       enabled_gl_platforms += 'cgl'
@@ -676,6 +675,7 @@ if need_platform_cgl != 'no'
     elif need_platform_cgl == 'yes'
       error('Could not find CGL dependencies')
     endif
+  endif
 endif
 
 # GDM Checks