Update flag forking approach to use split config.hpp and config_generated.hpp files

This commit reflects the new flag forking approach discussed in the
most recent Beman weekly sync:

- For any configuration option where a compiler flag could cause
  differences in behavior that lead to ODR issues when linking two
  artifacts generated with different flags, maintainers generate a
  compiler definition like BEMAN_EXEMPLAR_FOOBAR into a
  config_generated.hpp header from a config_generated.hpp.in template.
- Separately, a config.hpp file contains defaults for each
  definition. These defaults are only enabled if __has_include shows
  that the config_generated.hpp file is not present.

Author: ednolan

CMakeLists.txt

@@ -49,8 +49,8 @@ if(
 endif()
 
 configure_file(
-    "${PROJECT_SOURCE_DIR}/include/beman/iterator_interface/config.hpp.in"
-    "${PROJECT_BINARY_DIR}/include/beman/iterator_interface/config.hpp"
+    "${PROJECT_SOURCE_DIR}/include/beman/iterator_interface/config_generated.hpp.in"
+    "${PROJECT_BINARY_DIR}/include/beman/iterator_interface/config_generated.hpp"
     @ONLY
 )
 
@@ -67,7 +67,7 @@ target_sources(
         FILE_SET HEADERS
             BASE_DIRS
                 "${CMAKE_CURRENT_SOURCE_DIR}/include"
-                "${PROJECT_BINARY_DIR}/include"
+                "${CMAKE_CURRENT_BINARY_DIR}/include"
 )
 
 set_target_properties(

include/beman/iterator_interface/CMakeLists.txt

@@ -5,10 +5,11 @@ target_sources(
     PUBLIC
         FILE_SET HEADERS
             FILES
+                config.hpp
                 iterator_interface.hpp
                 iterator_interface_access.hpp
                 detail/stl_interfaces/config.hpp
                 detail/stl_interfaces/fwd.hpp
                 detail/stl_interfaces/iterator_interface.hpp
-                "${PROJECT_BINARY_DIR}/include/beman/iterator_interface/config.hpp"
+                "${PROJECT_BINARY_DIR}/include/beman/iterator_interface/config_generated.hpp"
 )

include/beman/iterator_interface/config.hpp

@@ -0,0 +1,11 @@
+#ifndef BEMAN_ITERATOR_INTERFACE_CONFIG_HPP
+#define BEMAN_ITERATOR_INTERFACE_CONFIG_HPP
+
+#if !defined(__has_include) ||                                                 \
+    __has_include(<beman/iterator_interface/config_generated.hpp>)
+#include <beman/iterator_interface/config_generated.hpp>
+#else
+#define BEMAN_ITERATOR_INTERFACE_USE_DEDUCING_THIS() 1
+#endif
+
+#endif

…e/beman/iterator_interface/config.hpp.in …erator_interface/config_generated.hpp.ininclude/beman/iterator_interface/config.hpp.in renamed to include/beman/iterator_interface/config_generated.hpp.in include/beman/iterator_interface/config.hpp.in renamed to include/beman/iterator_interface/config_generated.hpp.in

@@ -1,8 +1,8 @@
 // include/beman/iterator_interface/config.hpp.in -*-C++-*-
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#ifndef BEMAN_ITERATOR_INTERFACE_CONFIG_HPP
-#define BEMAN_ITERATOR_INTERFACE_CONFIG_HPP
+#ifndef BEMAN_ITERATOR_INTERFACE_CONFIG_GENERATED_HPP
+#define BEMAN_ITERATOR_INTERFACE_CONFIG_GENERATED_HPP
 
 #cmakedefine01 BEMAN_ITERATOR_INTERFACE_USE_DEDUCING_THIS()