Made the ODR rule violation error diagnostic clearer.

dlib/threads/threads_kernel_shared.cpp

@@ -9,7 +9,7 @@
 #include <iostream>
 
 // 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_ASSERTs enabled and part with them
+// 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"
 {

dlib/threads/threads_kernel_shared.h

@@ -13,11 +13,29 @@
 #include "../queue.h"
 #include "../set.h"
 
-// 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_ASSERTs enabled and part with them
-// disabled since doing that would be a violation of C++'s one definition rule. 
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 extern "C"
 {
+// =========================>>> WHY YOU ARE GETTING AN ERROR HERE <<<=========================
+// 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 it
+// disabled since doing that would be a violation of C++'s one definition rule.  So if you
+// are getting an error here then you are either not enabling DLIB_ASSERT consistently
+// (e.g. by compiling part of your program in a debug mode and part in a release mode) or
+// you have simply forgotten to compile dlib/all/source.cpp into your application.
+// =========================>>> WHY YOU ARE GETTING AN ERROR HERE <<<=========================
 #ifdef ENABLE_ASSERTS
     extern int USER_ERROR__violation_of_one_definition_rule_detected__inconsistent_use_of_DEBUG_or_ENABLE_ASSERTS_preprocessor_directives;
     inline int dlib_check_consistent_assert_usage() { USER_ERROR__violation_of_one_definition_rule_detected__inconsistent_use_of_DEBUG_or_ENABLE_ASSERTS_preprocessor_directives = 0; return 0; }
@@ -28,6 +46,19 @@ extern "C"
     const int dlib_check_assert_helper_variable = dlib_check_consistent_assert_usage();
 }
 
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 namespace dlib
 {