Fix the platform-detection condition in the PlatformGraphics module

We cannot use the fancy options in PlatformGraphics since user projects
lack of their evaluation in QtPlatformSupport.cmake. Use the explicit
CMAKE_SYSTEM_NAME-based conditions instead. Those should work if
respective CMAKE_SYSTEM_NAME is specified in the toolchain file.

Amends 3322f58

Change-Id: Ifb6ced360e8cade7315b2b238837578bfc9a1bc3
Reviewed-by: Kimmo Ollila <kimmo.ollila@qt.io>
Reviewed-by: Karim Pinter <karim.pinter@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from commit 43cac6e)
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/platforms/FindPlatformGraphics.cmake

@@ -5,9 +5,9 @@
 # PlatformGraphics
 # ---------
 
-if(INTEGRITY)
+if(CMAKE_SYSTEM_NAME STREQUAL "Integrity")
     set(platform Integrity)
-elseif(VXWORKS)
+elseif(CMAKE_SYSTEM_NAME STREQUAL "VxWorks")
     set(platform VxWorks)
 else()
     set(PlatformGraphics_FOUND FALSE)