CMake: Fixes for generated config (#1212)

* CMake: Add option to set the package install dir.

* CMake: Fix generated config.

- `YAML_CPP_SHARED_LIBS_BUILT` should not be set with a `PATH_VAR` as it
would always evaluate to true.
- `YAML_CPP_LIBRARIES` should used the exported target name including
the namespace, but `check_required_components` shouldn't.
- Use `CMAKE_CURRENT_LIST_DIR` to find the target file, instead of a
`PATH_VAR`. Package managers such as vcpkg move CMake configs after
installing.

* CI: Test the generated CMake package.

* CMake: Create add a deprecated yaml-cpp target.

This target is meant to provide compatibility with versions prior to
0.8.0.

* CMake: mark the yaml-cpp target as IMPORTED.

---------

Co-authored-by: Jesse Beder <jbeder+github@gmail.com>

test/cmake/CMakeLists.txt

@@ -0,0 +1,19 @@
+cmake_minimum_required(VERSION 3.5)
+project(yaml-cpp-consumer LANGUAGES CXX)
+
+find_package(yaml-cpp CONFIG REQUIRED)
+get_target_property(LIBRARY_TYPE yaml-cpp::yaml-cpp TYPE)
+
+if(LIBRARY_TYPE STREQUAL "SHARED_LIBRARY")
+  if(NOT YAML_CPP_SHARED_LIBS_BUILT)
+    message(FATAL_ERROR "Library type (${LIBRARY_TYPE}) contradicts config: ${YAML_CPP_SHARED_LIBS_BUILT}")
+  endif()
+else()
+  if(YAML_CPP_SHARED_LIBS_BUILT)
+    message(FATAL_ERROR "Library type (${LIBRARY_TYPE}) contradicts config: ${YAML_CPP_SHARED_LIBS_BUILT}")
+  endif()
+endif()
+
+add_executable(main main.cpp)
+set_target_properties(main PROPERTIES CXX_STANDARD 11)
+target_link_libraries(main PRIVATE ${YAML_CPP_LIBRARIES})

test/cmake/main.cpp

@@ -0,0 +1,3 @@
+#include "yaml-cpp/yaml.h"
+
+int main(int, char**) { YAML::Parser foo{}; }