correct grammar on UNIX missing dependency status notice.

CMakeLists.txt

@@ -1189,9 +1189,9 @@ message(STATUS " Build Shared Library: ${SDL_SHARED}")
 message(STATUS " Build Static Library: ${SDL_STATIC}")
 message(STATUS "")
 if(UNIX)
-  message(STATUS "Something was not detected, although your are sure that it")
-  message(STATUS "has been properly installed? Then make sure, you have set")
-  message(STATUS "the CFLAGS and LDFLAGS environment variables correctly.")
+  message(STATUS "If something was not detected, although the libraries")
+  message(STATUS "were installed, then make sure you have set the")
+  message(STATUS "CFLAGS and LDFLAGS environment variables correctly.")
   message(STATUS "")
 endif(UNIX)