Fix header file installation paths to respect CMAKE_INSTALL_INCLUDEDIR

Previously, header files were installed uniformly under the include directory, ignoring the specified paths.
This change ensures header files are installed in the appropriate directories under `CMAKE_INSTALL_INCLUDEDIR`.

CMakeLists.txt

@@ -253,6 +253,10 @@ IF (NOT DEFINED CMAKE_INSTALL_LIBDIR)
     SET(CMAKE_INSTALL_LIBDIR lib)
 ENDIF ()
 
+IF (NOT CMAKE_INSTALL_INCLUDEDIR)
+    SET(CMAKE_INSTALL_INCLUDEDIR include)
+ENDIF ()
+
 IF (MSGPACK_BUILD_EXAMPLES)
     ADD_SUBDIRECTORY (example)
 ENDIF ()
@@ -271,12 +275,14 @@ INSTALL (TARGETS ${MSGPACK_INSTALLTARGETS} EXPORT msgpack-c-targets
     ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
 )
 FOREACH (file ${msgpack-c_common_HEADERS})
-    GET_FILENAME_COMPONENT (dir ${file} PATH)
-    INSTALL (FILES ${file} DESTINATION ${dir})
+    GET_FILENAME_COMPONENT(dir ${file} DIRECTORY)
+    STRING(REPLACE "include" "${CMAKE_INSTALL_INCLUDEDIR}" header_path ${dir})
+    INSTALL (FILES ${file} DESTINATION ${header_path})
 ENDFOREACH ()
 FOREACH (file ${msgpack-c_configured_HEADERS})
-    GET_FILENAME_COMPONENT (dir ${file} PATH)
-    INSTALL (FILES ${CMAKE_CURRENT_BINARY_DIR}/${file} DESTINATION ${dir})
+    GET_FILENAME_COMPONENT(dir ${file} DIRECTORY)
+    STRING(REPLACE "include" "${CMAKE_INSTALL_INCLUDEDIR}" header_path ${dir})
+    INSTALL (FILES ${CMAKE_CURRENT_BINARY_DIR}/${file} DESTINATION ${header_path})
 ENDFOREACH ()
 IF (NOT MSVC)
     INSTALL (FILES ${CMAKE_CURRENT_BINARY_DIR}/msgpack-c.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)