Export GLFW targets

[aWilson41]

The current cmake code does not export its targets (modern cmake).

If you make a project that uses GLFW with the following CMakeListst.txt I would've expected it to work.

cmake_minimum_required(VERSION 3.16)
project(VulkanExample)

set(CMAKE_CXX_STANDARD 17)
find_package(Vulkan REQUIRED)
find_package(glfw3 REQUIRED)

add_executable(${PROJECT_NAME} main.cpp)

target_link_libraries(${PROJECT_NAME} Vulkan::Vulkan glfw3::glfw)

With the small change in this PR, the above is possible (without a Findglfw3.cmake file), only specify glfw3_DIR=/src.

Details:

The old cmake solution would've been to write a find file for GLFW. Findglfw3.cmake. But modern cmake makes life easier. GLFW should instead produce glfw3Config.cmake file (already done in GLFW) that imports its own targets. No need to write a find. But this shouldn't effect finds in any way.

When find_package is run it will search glfw3_DIR, cmake path, & system path for a find (old cmake, or non-cmake libs) OR config file (modern).

GLFW config file is produced in /src. When glfw3_DIR=/src is given in a project. It fails to find any targets in the config. Looking at GLFW cmake code, no targets are exported. This PR is a fix for that. It now exports targets and the above cmake should work simply by providing glfw3_DIR=/src. No need for a find.

[gracicot]

For the records, glfw do export targets. When you install glfw, it correctly writes glfw3Config and all. What you describe and what your PR adds is support for exporting from the build tree.