Ensure exported package is relocatable

As per CMake's Importing and Exporting Guide[1],
configure_package_config_file() should be used for configuring the
package configuration file, not the regular configure_file() function.

This ensures that a spdlog package built on one system (with a given
directory tree) can be imported from a different system -
e.g. creating a pre-compiled spdlog package for use on different
systems.

[1]: https://cmake.org/cmake/help/git-stage/guide/importing-exporting/index.html#id8

cmake/spdlogConfig.cmake.in

@@ -1,6 +1,8 @@
 # Copyright(c) 2019 spdlog authors
 # Distributed under the MIT License (http://opensource.org/licenses/MIT)
 
+@PACKAGE_INIT@
+
 find_package(Threads REQUIRED)
 
 set(SPDLOG_FMT_EXTERNAL @SPDLOG_FMT_EXTERNAL@)
@@ -13,3 +15,5 @@ endif()
 
 
 include("${CMAKE_CURRENT_LIST_DIR}/${config_targets_file}")
+
+check_required_components(spdlog)