BUG: Don't add -I/usr/include as an X_CFLAGS setting.

Modules/CMakeBackwardCompatibilityC.cmake

@@ -28,8 +28,12 @@ IF(NOT CMAKE_SKIP_COMPATIBILITY_TESTS)
   TEST_BIG_ENDIAN(CMAKE_WORDS_BIGENDIAN)
   INCLUDE (${CMAKE_ROOT}/Modules/FindX11.cmake)
 
-  SET (CMAKE_X_CFLAGS "-I${CMAKE_X11_INCLUDE_PATH}" CACHE STRING
-       "X11 extra flags.")
+  IF("${CMAKE_X11_INCLUDE_PATH}" MATCHES "^/usr/include$")
+    SET (CMAKE_X_CFLAGS "" CACHE STRING "X11 extra flags.")
+  ELSE("${CMAKE_X11_INCLUDE_PATH}" MATCHES "^/usr/include$")
+    SET (CMAKE_X_CFLAGS "-I${CMAKE_X11_INCLUDE_PATH}" CACHE STRING
+         "X11 extra flags.")
+  ENDIF("${CMAKE_X11_INCLUDE_PATH}" MATCHES "^/usr/include$")
   SET (CMAKE_X_LIBS "${CMAKE_X_LIBS}" CACHE STRING
        "Libraries and options used in X11 programs.")
   SET (CMAKE_HAS_X "${CMAKE_HAS_X}" CACHE INTERNAL "Is X11 around.")