Fix CMake export files (#1077)

After configuring the file `yaml-cpp-config.cmake.in`, the result ends up with
empty variables.  (see also the discussion in #774).

Rework this file and the call to `configure_package_config_file` according the
cmake documentation
(https://cmake.org/cmake/help/v3.22/module/CMakePackageConfigHelpers.html?highlight=configure_package_config#command:configure_package_config_file)
to overcome this issue and allow a simple `find_package` after install.

As there was some discussion about the place where to install the
`yaml-cpp-config.cmake` file, e.g. #1055, factor out the install location into
an extra variable to make it easier changing this location in the future.

Also untabify CMakeLists.txt in some places to align with the other code parts in this file.

CMakeLists.txt

@@ -133,10 +133,16 @@ set_target_properties(yaml-cpp PROPERTIES
   PROJECT_LABEL "yaml-cpp ${yaml-cpp-label-postfix}"
   DEBUG_POSTFIX "${CMAKE_DEBUG_POSTFIX}")
 
+# FIXME(felix2012): A more common place for the cmake export would be
+# `CMAKE_INSTALL_LIBDIR`, as e.g. done in ubuntu or in this project for GTest
+set(CONFIG_EXPORT_DIR "${CMAKE_INSTALL_DATADIR}/cmake/yaml-cpp")
+set(EXPORT_TARGETS yaml-cpp)
 configure_package_config_file(
   "${PROJECT_SOURCE_DIR}/yaml-cpp-config.cmake.in"
   "${PROJECT_BINARY_DIR}/yaml-cpp-config.cmake"
-  INSTALL_DESTINATION "${CMAKE_INSTALL_DATADIR}/cmake/yaml-cpp")
+  INSTALL_DESTINATION "${CONFIG_EXPORT_DIR}"
+  PATH_VARS CMAKE_INSTALL_INCLUDEDIR CONFIG_EXPORT_DIR)
+unset(EXPORT_TARGETS)
 
 write_basic_package_version_file(
   "${PROJECT_BINARY_DIR}/yaml-cpp-config-version.cmake"
@@ -145,30 +151,31 @@ write_basic_package_version_file(
 configure_file(yaml-cpp.pc.in yaml-cpp.pc @ONLY)
 
 if (YAML_CPP_INSTALL)
-	install(TARGETS yaml-cpp
+  install(TARGETS yaml-cpp
     EXPORT yaml-cpp-targets
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
     LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
     ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
-	install(DIRECTORY ${PROJECT_SOURCE_DIR}/include/
+  install(DIRECTORY ${PROJECT_SOURCE_DIR}/include/
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
-		FILES_MATCHING PATTERN "*.h")
+                FILES_MATCHING PATTERN "*.h")
   install(EXPORT yaml-cpp-targets
-    DESTINATION "${CMAKE_INSTALL_DATADIR}/cmake/yaml-cpp")
-	install(FILES
-		"${PROJECT_BINARY_DIR}/yaml-cpp-config.cmake"
-		"${PROJECT_BINARY_DIR}/yaml-cpp-config-version.cmake"
-    DESTINATION "${CMAKE_INSTALL_DATADIR}/cmake/yaml-cpp")
+    DESTINATION "${CONFIG_EXPORT_DIR}")
+  install(FILES
+      "${PROJECT_BINARY_DIR}/yaml-cpp-config.cmake"
+      "${PROJECT_BINARY_DIR}/yaml-cpp-config-version.cmake"
+    DESTINATION "${CONFIG_EXPORT_DIR}")
   install(FILES "${PROJECT_BINARY_DIR}/yaml-cpp.pc"
     DESTINATION ${CMAKE_INSTALL_DATADIR}/pkgconfig)
 endif()
+unset(CONFIG_EXPORT_DIR)
 
 if(YAML_CPP_BUILD_TESTS)
-	add_subdirectory(test)
+  add_subdirectory(test)
 endif()
 
 if(YAML_CPP_BUILD_TOOLS)
-	add_subdirectory(util)
+  add_subdirectory(util)
 endif()
 
 if (YAML_CPP_CLANG_FORMAT_EXE)

yaml-cpp-config.cmake.in

@@ -3,12 +3,14 @@
 #  YAML_CPP_INCLUDE_DIR - include directory
 #  YAML_CPP_LIBRARIES    - libraries to link against
 
-# Compute paths
-get_filename_component(YAML_CPP_CMAKE_DIR "${CMAKE_CURRENT_LIST_FILE}" PATH)
-set(YAML_CPP_INCLUDE_DIR "@CONFIG_INCLUDE_DIRS@")
+@PACKAGE_INIT@
+
+set_and_check(YAML_CPP_INCLUDE_DIR "@PACKAGE_CMAKE_INSTALL_INCLUDEDIR@")
 
 # Our library dependencies (contains definitions for IMPORTED targets)
-include("${YAML_CPP_CMAKE_DIR}/yaml-cpp-targets.cmake")
+include(@PACKAGE_CONFIG_EXPORT_DIR@/yaml-cpp-targets.cmake)
 
 # These are IMPORTED targets created by yaml-cpp-targets.cmake
 set(YAML_CPP_LIBRARIES "@EXPORT_TARGETS@")
+
+check_required_components(@EXPORT_TARGETS@)