Make godot-cpp installable with cmake config

This is so this library can be used via a package manager

The install destination uses CMAKE_INSTALL_ so that package managers
can choose the best location for these artifacts

godot-cpp-dev is the installed component this is so that if this
library is a subproject, the user can install their project without
installing this library by specifying the components

As BUILD_INTERFACE requires absolute path, this means that
GODOT_GDEXTENSION_DIR needs to be an absolute path

config filename can either be PascalCase or kebab-case, the latter was
chosen to be consistent with the package's name (godot-cpp)

string(JSON ...) is only available in cmake v3.19, as it is used to
obtain the version, this means that config-version file is only created
when using cmake v3.19 or later

CMakeLists.txt

@@ -30,6 +30,10 @@
 #		-DANDROID_TOOLCHAIN_NAME=arm-linux-androideabi-4.9 -DANDROID_PLATFORM=android-23 -DCMAKE_BUILD_TYPE=Debug .
 # cmake --build .
 #
+# Installation (after the library is built):
+# cmake --install <insert_build_directory_here> --component godot-cpp-dev
+#
+#
 # Protip
 # Generate the buildfiles in a sub directory to not clutter the root directory with build files:
 # mkdir build && cd build && cmake -G "Unix Makefiles" .. && cmake --build .
@@ -65,9 +69,8 @@ if(NOT DEFINED BITS)
 endif()
 
 # Input from user for GDExtension interface header and the API JSON file
-set(GODOT_GDEXTENSION_DIR "gdextension" CACHE STRING "")
+set(GODOT_GDEXTENSION_DIR "${CMAKE_CURRENT_SOURCE_DIR}/gdextension" CACHE STRING "")
 set(GODOT_CUSTOM_API_FILE "" CACHE STRING "")
-
 set(GODOT_GDEXTENSION_API_FILE "${GODOT_GDEXTENSION_DIR}/extension_api.json")
 if (NOT "${GODOT_CUSTOM_API_FILE}" STREQUAL "")  # User-defined override.
 	set(GODOT_GDEXTENSION_API_FILE "${GODOT_CUSTOM_API_FILE}")
@@ -183,9 +186,10 @@ if (GODOT_CPP_SYSTEM_HEADERS)
 endif ()
 
 target_include_directories(${PROJECT_NAME} ${GODOT_CPP_SYSTEM_HEADERS_ATTRIBUTE} PUBLIC
-	include
-	${CMAKE_CURRENT_BINARY_DIR}/gen/include
-	${GODOT_GDEXTENSION_DIR}
+	$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+	$<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/gen/include>
+	$<BUILD_INTERFACE:${GODOT_GDEXTENSION_DIR}>
+	$<INSTALL_INTERFACE:include>
 )
 
 # Add the compile flags
@@ -213,4 +217,8 @@ set_target_properties(${PROJECT_NAME}
 		LIBRARY_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin"
 		RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin"
 		OUTPUT_NAME "${OUTPUT_NAME}"
+		EXPORT_NAME "cpp"	# This ensures that the exported target is godot::cpp
 )
+
+
+include("cmake/install.cmake")

cmake/config.cmake

@@ -0,0 +1 @@
+include("${CMAKE_CURRENT_LIST_DIR}/godot-cpp-target.cmake")

cmake/install.cmake

@@ -0,0 +1,67 @@
+
+include("CMakePackageConfigHelpers")
+include("GNUInstallDirs")
+
+# Install the library and headers to their respective install location
+# CMAKE_INSTALL_ are used to allow the package manager to chose the install location
+# Components are used so that if godot-cpp is a subproject, the user can chose not to install it
+install(TARGETS "godot-cpp"
+	EXPORT "godot-cpp-target"
+	ARCHIVE
+		DESTINATION "${CMAKE_INSTALL_LIBDIR}"
+		COMPONENT "godot-cpp-dev"
+)
+install(
+	DIRECTORY
+		"${CMAKE_CURRENT_SOURCE_DIR}/include/"
+		"${CMAKE_CURRENT_BINARY_DIR}/gen/include/"
+	DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
+	COMPONENT "godot-cpp-dev"
+)
+# Install the gdextension files
+# The gdextension header is assumed to be the root include directory
+# As the JSON file is neither a header nor lib file it goes to the datadir
+install(FILES "${GODOT_GDEXTENSION_DIR}/gdextension_interface.h"
+	DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
+	COMPONENT "godot-cpp-dev"
+)
+install(FILES "${GODOT_GDEXTENSION_API_FILE}"
+	DESTINATION "${CMAKE_INSTALL_DATADIR}/godot-cpp"
+	COMPONENT "godot-cpp-dev"
+)
+
+# Install the export config file
+# This allows this library to be easily consumed by cmake projects:
+#	find_package("godot-cpp" 4.2.0 CONFIG REQUIRED)
+#	target_link_libaries("my-project" PRIVATE "godot::cpp")
+install(FILES "${CMAKE_CURRENT_SOURCE_DIR}/cmake/config.cmake"
+	RENAME "godot-cpp-config.cmake"
+	DESTINATION "${CMAKE_INSTALL_DATADIR}/godot-cpp"
+	COMPONENT "godot-cpp-dev"
+)
+install(EXPORT "godot-cpp-target"
+	NAMESPACE "godot::"
+	DESTINATION "${CMAKE_INSTALL_DATADIR}/godot-cpp"
+	COMPONENT "godot-cpp-dev"
+)
+
+if(CMAKE_VERSION VERSION_GREATER_EQUAL "3.19") # string(JSON...) only available in cmake v3.19+
+	# Use the JSON api file to get the version
+	file(READ "${GODOT_GDEXTENSION_API_FILE}" GODOT_GDEXTENSION_API_JSON)
+	# GODOT_API_VERSION_MAJOR = GODOT_GDEXTENSION_API_JSON["header"]["version_major"]
+	string(JSON GODOT_API_VERSION_MAJOR GET "${GODOT_GDEXTENSION_API_JSON}" "header" "version_major")
+	string(JSON GODOT_API_VERSION_MINOR GET "${GODOT_GDEXTENSION_API_JSON}" "header" "version_minor")
+	string(JSON GODOT_API_VERSION_PATCH GET "${GODOT_GDEXTENSION_API_JSON}" "header" "version_patch")
+	set(GODOT_API_VERSION "${GODOT_API_VERSION_MAJOR}.${GODOT_API_VERSION_MINOR}.${GODOT_API_VERSION_PATCH}")
+	unset(GODOT_GDEXTENSION_API_JSON)
+
+	# Install the config version file so that the gdextension version can be specified in find_package
+	write_basic_package_version_file("${CMAKE_CURRENT_BINARY_DIR}/godot-cpp-config-version.cmake"
+		VERSION "${GODOT_API_VERSION}"
+		COMPATIBILITY SameMinorVersion
+	)
+	install(FILES "${CMAKE_CURRENT_BINARY_DIR}/godot-cpp-config-version.cmake"
+		DESTINATION "${CMAKE_INSTALL_DATADIR}/godot-cpp"
+		COMPONENT "godot-cpp-dev"
+	)
+endif()