Made top level cmake file not build a shared library if part of a subproject.

CMakeLists.txt

 cmake_minimum_required(VERSION 2.8.12)
 
-# Set this so that the dlib subfolder will build both a static and shared
-# library, since the only reason to build this CMakeLists.txt is if you want
-# to install dlib.  It should be noted however that installing dlib is not
-# necessary.  A simpler approach is to use it the way shown in
-# examples/CMakeLists.txt 
-set(DLIB_IN_PROJECT_BUILD false)
+
+
+get_directory_property(has_parent PARENT_DIRECTORY)
+if(NOT has_parent)
+   # Set this so that the dlib subfolder will build both a static and shared
+   # library, since the only reason to build this CMakeLists.txt by itself is
+   # if you want to install dlib.  It should be noted however that installing
+   # dlib is not necessary.  A simpler approach is to use it the way shown in
+   # examples/CMakeLists.txt 
+   set(DLIB_IN_PROJECT_BUILD false)
+endif()
 
 add_subdirectory(dlib)