Added cmake package config so other packages can find ALE (#294)

* Updated install command

* Updated so number

* Added target export

* Actually exported the target

* Figured out what the config does

CMakeLists.txt

@@ -23,14 +23,17 @@ find_package(nlohmann_json REQUIRED)
 # Library setup
 add_library(ale SHARED
             ${CMAKE_CURRENT_SOURCE_DIR}/src/ale.cpp)
+# Alias a scoped target for safer linking in downstream projects
+set(ALE_HEADERS "include/ale.h")
 set_target_properties(ale PROPERTIES
                       VERSION       ${PROJECT_VERSION}
-    SOVERSION 1)
-set(ALE_INCLUDE_DIRS "${CMAKE_CURRENT_SOURCE_DIR}/include/")
-
+                      SOVERSION     0
+                      PUBLIC_HEADER ${ALE_HEADERS})
+# Use generator expressions so that downstream projects can use this target
 target_include_directories(ale
                            PUBLIC
-                           ${ALE_INCLUDE_DIRS})
+                           $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+                           $<INSTALL_INTERFACE:include>)
 
 target_link_libraries(ale
                       PRIVATE
@@ -60,6 +63,22 @@ if(BUILD_TESTS)
     endif()
 endif()
 
-# Install commands
-install(TARGETS ale LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR})
-install(DIRECTORY ${ALE_INCLUDE_DIRS} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+# Generate the package config
+configure_file(cmake/config.cmake.in
+               ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}-config.cmake
+               @ONLY)
+
+# Install the package config
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}-config.cmake
+              DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME})
+
+# Install the library
+install(TARGETS ale
+        EXPORT aleTargets
+        LIBRARY       DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        PUBLIC_HEADER DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+
+# Install the target
+install(EXPORT aleTargets
+        NAMESPACE ale::
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME})

cmake/config.cmake.in

+include(FindPackageHandleStandardArgs)
+set(${CMAKE_FIND_PACKAGE_NAME}_CONFIG ${CMAKE_CURRENT_LIST_FILE})
+find_package_handle_standard_args(@PROJECT_NAME@ CONFIG_MODE)
+
+if(NOT TARGET @PROJECT_NAME@::ale)
+    include("${CMAKE_CURRENT_LIST_DIR}/aleTargets.cmake")
+endif()