Fixed the cmake option to toggle the ENABLE_ASSERTS macro

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402479

dlib/CMakeLists.txt

@@ -165,8 +165,8 @@ endif()
 
 
 if (DLIB_ENABLE_ASSERTS)
-   add_global_define(DLIB_ENABLE_ASSERTS)
+   add_global_define(ENABLE_ASSERTS)
 else()
-   remove_global_define(DLIB_ENABLE_ASSERTS)
+   remove_global_define(ENABLE_ASSERTS)
 endif()