Made cmake file a little more convenient when used in large projects.

dlib/cmake

 
+# Don't add dlib if it's already been added to the cmake project
+if (NOT TARGET dlib)
 
-# Determine the path to dlib.
-string(REPLACE "cmake" "" dlib_path ${CMAKE_CURRENT_LIST_FILE})
+    # Determine the path to dlib.
+    string(REPLACE "cmake" "" dlib_path ${CMAKE_CURRENT_LIST_FILE})
 
-if (CMAKE_COMPILER_IS_GNUCXX)
+    if (CMAKE_COMPILER_IS_GNUCXX)
         # By default, g++ won't warn or error if you forget to return a value in a
         # function which requires you to do so.  This option makes it give a warning
         # for doing this. 
         add_definitions("-Wreturn-type")
-endif()
+    endif()
+
+    # Add folder containing dlib to the include search path.
+    INCLUDE_DIRECTORIES(${dlib_path}/..)
 
-# Add folder containing dlib to the include search path.
-INCLUDE_DIRECTORIES(${dlib_path}/..)
+    # This is really optional, but nice.  It will make sure the build mode 
+    # created by cmake is always release by default.
+    include(${dlib_path}/release_build_by_default)
 
-# This is really optional, but nice.  It will make sure the build mode 
-# created by cmake is always release by default.
-include(${dlib_path}/release_build_by_default)
+    add_subdirectory(${dlib_path} dlib_build)
+endif()
 
-add_subdirectory(${dlib_path} dlib_build)