CMake: use correct Raspberry Pi library names & fix generic GLES support

* For BCMHOST build, use brcmEGL / brcmGLESv2 libraries, and don't look at
  Mesa includes (so libraspberrypi-dev and libgles*-mesa-dev can coexist).
* If overridden via -DGLES=On, don't present vendor includes to build to
  make absolutely sure that the Mesa includes are used.

Fixes stretch vendor library building & generic Mesa GLES compatibility.

CMake/Packages/FindOpenGLES.cmake

@@ -42,11 +42,23 @@ ELSE (WIN32)
 
   ELSE(APPLE)
 
+    IF (DEFINED BCMHOST)
+      FIND_PATH(OPENGLES_INCLUDE_DIR GLES/gl.h
+        /opt/vc/include
+        NO_DEFAULT_PATH
+      )
+
+      FIND_LIBRARY(OPENGLES_gl_LIBRARY
+        NAMES brcmGLESv2
+        PATHS /opt/vc/lib
+      )
+
+    ELSE (DEFINED BCMHOST)
+
       FIND_PATH(OPENGLES_INCLUDE_DIR GLES/gl.h
         /usr/openwin/share/include
         /opt/graphics/OpenGL/include /usr/X11R6/include
         /usr/include
-      /opt/vc/include
       )
 
       FIND_LIBRARY(OPENGLES_gl_LIBRARY
@@ -55,8 +67,8 @@ ELSE (WIN32)
               /usr/openwin/lib
               /usr/shlib /usr/X11R6/lib
               /usr/lib
-            /opt/vc/lib
       )
+    ENDIF (DEFINED BCMHOST)
 
     # On Unix OpenGL most certainly always requires X11.
     # Feel free to tighten up these conditions if you don't 

CMakeLists.txt

@@ -178,7 +178,7 @@ endif()
 if(DEFINED BCMHOST)
     LIST(APPEND COMMON_LIBRARIES
         bcm_host
-        EGL
+        brcmEGL
         ${OPENGLES_LIBRARIES}
     )
 else()