cmake: improve CMakeLists.txt

* support CMake user registry package registration
* use GNUInstallDirs to set installation firectories
* use spdlog namespace in both build and install interfaces

CMakeLists.txt

-#
-# Copyright(c) 2015 Ruslan Baratov.
-# Distributed under the MIT License (http://opensource.org/licenses/MIT)
-#
-
-cmake_minimum_required(VERSION 3.1)
-project(spdlog VERSION 1.0.0)
-include(CTest)
-include(CMakeDependentOption)
-
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-
-if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
-    set(CMAKE_CXX_FLAGS "-Wall ${CMAKE_CXX_FLAGS}")
-endif()
-
-add_library(spdlog INTERFACE)
-
-option(SPDLOG_BUILD_EXAMPLES "Build examples" OFF)
-cmake_dependent_option(SPDLOG_BUILD_TESTING 
-    "Build spdlog tests" ON
-    "BUILD_TESTING" OFF
-)
-
-target_include_directories(
-    spdlog
-    INTERFACE
-    "$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>"
-    "$<INSTALL_INTERFACE:include>"
-)
-
-set(HEADER_BASE "${CMAKE_CURRENT_SOURCE_DIR}/include")
-
-if(SPDLOG_BUILD_EXAMPLES)
-    add_subdirectory(example)
-endif()
-
-if(SPDLOG_BUILD_TESTING)
-    add_subdirectory(tests)
-endif()
-
 ### Install ###
-# * https://github.com/forexample/package-example
-set(generated_dir "${CMAKE_CURRENT_BINARY_DIR}/generated")
-
-set(config_install_dir "lib/cmake/${PROJECT_NAME}")
-set(include_install_dir "include")
-set(pkgconfig_install_dir "lib/pkgconfig")
-
-set(version_config "${generated_dir}/${PROJECT_NAME}ConfigVersion.cmake")
-set(project_config "${generated_dir}/${PROJECT_NAME}Config.cmake")
-set(pkg_config "${generated_dir}/${PROJECT_NAME}.pc")
-set(targets_export_name "${PROJECT_NAME}Targets")
-set(namespace "${PROJECT_NAME}::")
-
-include(CMakePackageConfigHelpers)
-write_basic_package_version_file(
-    "${version_config}" COMPATIBILITY SameMajorVersion
-)
-
 # Note: use 'targets_export_name'
-configure_file("cmake/Config.cmake.in" "${project_config}" @ONLY)
-configure_file("cmake/spdlog.pc.in" "${pkg_config}" @ONLY)
-
-install(
-    TARGETS spdlog
-    EXPORT "${targets_export_name}"
-    INCLUDES DESTINATION "${include_install_dir}"
-)
-
-install(DIRECTORY "include/spdlog" DESTINATION "${include_install_dir}")
-
-install(
-    FILES "${project_config}" "${version_config}"
-    DESTINATION "${config_install_dir}"
-)
-
-install(
-    FILES "${pkg_config}"
-    DESTINATION "${pkgconfig_install_dir}"
-)
-
-install(
-    EXPORT "${targets_export_name}"
-    NAMESPACE "${namespace}"
-    DESTINATION "${config_install_dir}"
-)
-
-file(GLOB_RECURSE spdlog_include_SRCS "${HEADER_BASE}/*.h")
-add_custom_target(spdlog_headers_for_ide SOURCES ${spdlog_include_SRCS})
+#
+# Copyright(c) 2015 Ruslan Baratov.
+# Distributed under the MIT License (http://opensource.org/licenses/MIT)
+#
+
+cmake_minimum_required(VERSION 3.1)
+project(spdlog VERSION 1.0.0)
+include(CTest)
+include(CMakeDependentOption)
+include(GNUInstallDirs)
+
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+    set(CMAKE_CXX_FLAGS "-Wall ${CMAKE_CXX_FLAGS}")
+endif()
+
+add_library(spdlog INTERFACE)
+
+option(SPDLOG_BUILD_EXAMPLES "Build examples" OFF)
+cmake_dependent_option(SPDLOG_BUILD_TESTING
+    "Build spdlog tests" ON
+    "BUILD_TESTING" OFF
+)
+
+target_include_directories(
+    spdlog
+    INTERFACE
+    "$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>"
+    "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
+)
+
+set(HEADER_BASE "${CMAKE_CURRENT_SOURCE_DIR}/include")
+
+if(SPDLOG_BUILD_EXAMPLES)
+    add_subdirectory(example)
+endif()
+
+if(SPDLOG_BUILD_TESTING)
+    add_subdirectory(tests)
+endif()
+
+set(config_install_dir "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}")
+set(include_install_dir "${CMAKE_INSTALL_INCLUDEDIR}")
+set(pkgconfig_install_dir "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+set(version_config "${CMAKE_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake")
+set(project_config "${PROJECT_NAME}Config.cmake")
+set(pkg_config "${CMAKE_BINARY_DIR}/${PROJECT_NAME}.pc")
+set(targets_export_name "${PROJECT_NAME}Targets")
+set(namespace "${PROJECT_NAME}::")
+
+include(CMakePackageConfigHelpers)
+write_basic_package_version_file(
+    "${version_config}" COMPATIBILITY SameMajorVersion
+)
+
+configure_file("cmake/spdlog.pc.in" "${pkg_config}" @ONLY)
+
+install(
+    TARGETS spdlog
+    EXPORT "${targets_export_name}"
+    INCLUDES DESTINATION "${include_install_dir}"
+)
+
+install(
+    DIRECTORY "${HEADER_BASE}/${PROJECT_NAME}"
+    DESTINATION "${include_install_dir}"
+)
+
+install(
+    FILES "${version_config}"
+    DESTINATION "${config_install_dir}"
+)
+
+install(
+    FILES "${pkg_config}"
+    DESTINATION "${pkgconfig_install_dir}"
+)
+
+install(
+    EXPORT "${targets_export_name}"
+    NAMESPACE "${namespace}"
+    DESTINATION "${config_install_dir}"
+    FILE ${project_config}
+)
+
+export(
+    EXPORT ${targets_export_name}
+    NAMESPACE "${namespace}"
+    FILE ${project_config}
+)
+
+export(PACKAGE ${PROJECT_NAME})
+
+file(GLOB_RECURSE spdlog_include_SRCS "${HEADER_BASE}/*.h")
+add_custom_target(spdlog_headers_for_ide SOURCES ${spdlog_include_SRCS})