Added some code that will cause people who make a standalone dlib build and

then take the compiled library file but ignore the config.h file to get a
linker error telling them they have been naughty.  Instead, they should either
use `make install` or `cmake --build . --target install` which will copy the
correct files, or even better, don't install/build dlib as a standalone
library.  Instead, use it the way shown in examples/CMakeLists.txt.

dlib/CMakeLists.txt

@@ -739,6 +739,10 @@ if (NOT TARGET dlib)
       # contents of dlib/config.h once it's installed. But for in project
       # builds, there is no real config.h so they are public in the above case.
       target_compile_options(dlib PRIVATE ${active_preprocessor_switches})
+      # Do this so that dlib/config.h won't set DLIB_NOT_CONFIGURED. This will then allow
+      # the code in dlib/threads_kernel_shared.cpp to emit a linker error for users who
+      # don't use the configured config.h file generated by cmake.
+      target_compile_options(dlib PRIVATE -DDLIB__CMAKE_GENERATED_A_CONFIG_H_FILE)
    endif()
 
 

dlib/config.h

@@ -23,3 +23,9 @@
 // #define DLIB_USE_BLAS
 // #define DLIB_USE_LAPACK
 // #define DLIB_USE_CUDA
+
+
+#ifndef DLIB__CMAKE_GENERATED_A_CONFIG_H_FILE
+#define DLIB_NOT_CONFIGURED
+#endif 
+

dlib/threads/threads_kernel_shared.cpp

@@ -8,16 +8,32 @@
 #include "../platform.h"
 #include <iostream>
 
+extern "C"
+{
 // The point of this block of code is to cause a link time error that will prevent a user
 // from compiling part of their application with DLIB_ASSERT enabled and part with them
 // disabled since doing that would be a violation of C++'s one definition rule. 
-extern "C"
-{
 #ifdef ENABLE_ASSERTS
     int USER_ERROR__missing_dlib_all_source_cpp_file__OR__inconsistent_use_of_DEBUG_or_ENABLE_ASSERTS_preprocessor_directives;
 #else
     int USER_ERROR__missing_dlib_all_source_cpp_file__OR__inconsistent_use_of_DEBUG_or_ENABLE_ASSERTS_preprocessor_directives_;
 #endif
+
+
+// The point of this block of code is to cause a link time error if someone builds dlib via
+// cmake as a separately installable library, and therefore generates a dlib/config.h from
+// cmake, but then proceeds to use the default unconfigured dlib/config.h from version
+// control.  It should be obvious why this is bad, if it isn't you need to read a book
+// about C++.  Moreover, it can only happen if someone manually copies files around and
+// messes things up.  If instead they run `make install` or `cmake --build .  --target
+// install` things will be setup correctly, which is what they should do.  To summarize: DO
+// NOT BUILD A STANDALONE DLIB AND THEN GO CHERRY PICKING FILES FROM THE BUILD FOLDER AND
+// MIXING THEM WITH THE SOURCE FROM GITHUB.  USE CMAKE'S INSTALL SCRIPTS TO INSTALL DLIB.
+// Or even better, don't install dlib at all and instead build your program as shown in
+// examples/CMakeLists.txt
+#ifdef DLIB_NOT_CONFIGURED
+    int USER_ERROR__missing_dlib_all_source_cpp_file__OR__inconsistent_build_configuration;
+#endif
 }
 
 #ifndef DLIB_THREAD_POOL_TIMEOUT

dlib/threads/threads_kernel_shared.h

@@ -44,6 +44,28 @@ extern "C"
     inline int dlib_check_consistent_assert_usage() { USER_ERROR__missing_dlib_all_source_cpp_file__OR__inconsistent_use_of_DEBUG_or_ENABLE_ASSERTS_preprocessor_directives_ = 0; return 0; }
 #endif
     const int DLIB_NO_WARN_UNUSED dlib_check_assert_helper_variable = dlib_check_consistent_assert_usage();
+
+
+
+// The point of this block of code is to cause a link time error if someone builds dlib via
+// cmake as a separately installable library, and therefore generates a dlib/config.h from
+// cmake, but then proceeds to use the default unconfigured dlib/config.h from version
+// control.  It should be obvious why this is bad, if it isn't you need to read a book
+// about C++.  Moreover, it can only happen if someone manually copies files around and
+// messes things up.  If instead they run `make install` or `cmake --build .  --target
+// install` things will be setup correctly, which is what they should do.  To summarize: DO
+// NOT BUILD A STANDALONE DLIB AND THEN GO CHERRY PICKING FILES FROM THE BUILD FOLDER AND
+// MIXING THEM WITH THE SOURCE FROM GITHUB.  USE CMAKE'S INSTALL SCRIPTS TO INSTALL DLIB.
+// Or even better, don't install dlib at all and instead build your program as shown in
+// examples/CMakeLists.txt
+#ifdef DLIB_NOT_CONFIGURED 
+    extern int USER_ERROR__missing_dlib_all_source_cpp_file__OR__inconsistent_build_configuration;
+    inline int dlib_check_consistent_config_h_usage() { USER_ERROR__missing_dlib_all_source_cpp_file__OR__inconsistent_build_configuration = 0; return 0; }
+#else
+    inline int dlib_check_consistent_config_h_usage() { return 0; }
+#endif
+    const int DLIB_NO_WARN_UNUSED dlib_check_not_configured_helper_variable = dlib_check_consistent_config_h_usage();
+
 }