cmake: add headers to interface library

cmake 3.19 allows headers to be added to interface libraries in order to
populate IDE projects

CMakeLists.txt

@@ -6,6 +6,11 @@ PROJECT (msgpack-cxx LANGUAGES CXX)
 
 ADD_LIBRARY (msgpack-cxx INTERFACE)
 
+IF (CMAKE_VERSION VERSION_GREATER_EQUAL 3.19)
+    FILE(GLOB_RECURSE SOURCE_FILES include/*hpp)
+    TARGET_SOURCES(msgpack-cxx PRIVATE ${SOURCE_FILES})
+ENDIF()
+
 FILE (READ ${CMAKE_CURRENT_SOURCE_DIR}/include/msgpack/version_master.hpp contents)
 STRING (REGEX MATCH "#define MSGPACK_VERSION_MAJOR *([0-9a-zA-Z_]*)" NULL_OUT ${contents})
 SET (VERSION_MAJOR ${CMAKE_MATCH_1})