Add option MSGPACK_BUILD_DOCS.

This option explicitly controls the generation of targets related to
Doxygen generation, rather than relying solely on whether Doxygen is
discovered.

It is enabled by default to preserve existing behavior, but if disabled
then no Doxygen targets will be generated. This is useful when the
library is included via CMake's `add_subdirectory()`.

Author: marascio

CMakeLists.txt

@@ -24,6 +24,7 @@ OPTION (MSGPACK_CXX20 "Using c++20 compiler" OFF)
 OPTION (MSGPACK_32BIT            "32bit compile"                        OFF)
 OPTION (MSGPACK_USE_X3_PARSE     "Use Boost X3 parse"                   OFF)
 OPTION (MSGPACK_BUILD_TESTS      "Build tests"                          OFF)
+OPTION (MSGPACK_BUILD_DOCS       "Build Doxygen documentation"          ON)
 OPTION (MSGPACK_FUZZ_REGRESSION  "Enable regression testing"            OFF)
 OPTION (MSGPACK_BUILD_EXAMPLES   "Build msgpack examples"               OFF)
 OPTION (MSGPACK_GEN_COVERAGE     "Generate coverage report"             OFF)
@@ -153,28 +154,30 @@ IF (MSGPACK_BUILD_EXAMPLES)
 ENDIF ()
 
 # Doxygen
-FIND_PACKAGE (Doxygen)
-IF (DOXYGEN_FOUND)
-    LIST (APPEND Doxyfile_cpp_CONTENT
-        COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
-        COMMAND ${CMAKE_COMMAND} -E echo "FILE_PATTERNS      = *.hpp" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
-        COMMAND ${CMAKE_COMMAND} -E echo "OUTPUT_DIRECTORY   = doc_cpp" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
-        COMMAND ${CMAKE_COMMAND} -E echo "INPUT              = ${CMAKE_CURRENT_SOURCE_DIR}/include" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
-        COMMAND ${CMAKE_COMMAND} -E echo "EXTRACT_ALL        = YES" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
-        COMMAND ${CMAKE_COMMAND} -E echo "STRIP_FROM_PATH    = ${CMAKE_CURRENT_SOURCE_DIR}/include" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
-    )
-    IF (DOXYGEN_DOT_FOUND)
+IF (MSGPACK_BUILD_DOCS)
+    FIND_PACKAGE (Doxygen)
+    IF (DOXYGEN_FOUND)
         LIST (APPEND Doxyfile_cpp_CONTENT
-            COMMAND ${CMAKE_COMMAND} -E echo "HAVE_DOT       = YES" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            COMMAND ${CMAKE_COMMAND} -E echo "FILE_PATTERNS      = *.hpp" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            COMMAND ${CMAKE_COMMAND} -E echo "OUTPUT_DIRECTORY   = doc_cpp" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            COMMAND ${CMAKE_COMMAND} -E echo "INPUT              = ${CMAKE_CURRENT_SOURCE_DIR}/include" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            COMMAND ${CMAKE_COMMAND} -E echo "EXTRACT_ALL        = YES" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            COMMAND ${CMAKE_COMMAND} -E echo "STRIP_FROM_PATH    = ${CMAKE_CURRENT_SOURCE_DIR}/include" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+        )
+        IF (DOXYGEN_DOT_FOUND)
+            LIST (APPEND Doxyfile_cpp_CONTENT
+                COMMAND ${CMAKE_COMMAND} -E echo "HAVE_DOT       = YES" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            )
+        ENDIF ()
+        ADD_CUSTOM_TARGET (
+            doxygen
+            ${Doxyfile_cpp_CONTENT}
+            COMMAND ${CMAKE_COMMAND} -E echo "PROJECT_NAME       = \"MessagePack for C++\"" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
+            VERBATIM
         )
     ENDIF ()
-    ADD_CUSTOM_TARGET (
-        doxygen
-        ${Doxyfile_cpp_CONTENT}
-        COMMAND ${CMAKE_COMMAND} -E echo "PROJECT_NAME       = \"MessagePack for C++\"" >> ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
-        COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile_cpp
-        VERBATIM
-    )
 ENDIF ()
 
 # Install library.