Add cmake package config support

Add support for CMake find_package command to msgpack. The package
exports the two targets msgpackc-static and msgpackc if shared is
enabled. The CMake find_package command works after installation
of msgpack and during build of another project which adds msgpack
as sub project via add_subdirectory command.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>

msgpack-config.cmake.in

@@ -0,0 +1,23 @@
+#.rst:
+# msgpack
+# -------
+#
+# The following import targets are created
+#
+# ::
+#
+#   msgpackc-static
+#   msgpackc
+#
+
+@PACKAGE_INIT@
+
+include(CMakeFindDependencyMacro)
+
+if(NOT TARGET msgpackc AND NOT TARGET msgpackc-static)
+  include("${CMAKE_CURRENT_LIST_DIR}/msgpack-targets.cmake")
+
+  if(NOT @MSGPACK_ENABLE_SHARED@)
+    add_library(msgpackc ALIAS msgpackc-static)
+  endif()
+endif()