chore: add preprocessor support for doxygen

doc/CMakeLists.txt

@@ -1,13 +1,31 @@
-# Configure Doxygen config file
+# Extract all public macros defined in YYCC
+# However, you should note that these extratcted macros have generator expressions.
+get_target_property(YYCC_COMPILE_DEFINITIONS YYCCommonplace COMPILE_DEFINITIONS)
+if (YYCC_COMPILE_DEFINITIONS STREQUAL "YYCC_COMPILE_DEFINITIONS-NOTFOUND")
+    message(FATAL_ERROR "Cannot extract compile definitions from YYCCommonplace.")
+endif ()
+# Convert list to string for expanding in future.
+list(JOIN YYCC_COMPILE_DEFINITIONS " " YYCC_MACRO_GENERATOR_EXPRESSIONS)
+
+# We simply configure Doxygen config file first.
 configure_file(
     ${CMAKE_CURRENT_LIST_DIR}/Doxyfile.in
     ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
     @ONLY
 )
 
-# Add custom target
+# Then we use "file GENERATE" syntax to generate per-config truely Doxyfile used by Doxygen.
+# Because there is no "$<>" syntax in Doxyfile, so we can safely use it.
+# Please note that the generation of "file GENERATE" syntax will be postponed until the build stage.
+file(GENERATE
+    OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/$<CONFIG>/Doxyfile"
+    INPUT "${CMAKE_CURRENT_BINARY_DIR}/Doxyfile"
+    TARGET YYCCommonplace
+)
+
+# Add custom target using per-config Doxyfile
 add_custom_target (YYCCDocumentation
-    doxygen ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
+    Doxygen::doxygen "${CMAKE_CURRENT_BINARY_DIR}/$<CONFIG>/Doxyfile"
     WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
     COMMENT "Generating documentation" VERBATIM
 )