Only INSTALL_INTERFACE for static.

[vrabaud]

This fixes #2264

[wantehchang]

I also wanted to note that we can't test this code (lines 172-174) right now, because in a static build, the cmake config files are not installed:

    # Enable CMake configs in VCPKG mode
    if(BUILD_SHARED_LIBS OR VCPKG_TARGET_TRIPLET)   
        install(EXPORT ${PROJECT_NAME}-config DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME})
        
        include(CMakePackageConfigHelpers)
        write_basic_package_version_file(
            ${PROJECT_NAME}-config-version.cmake VERSION ${PROJECT_VERSION} COMPATIBILITY SameMajorVersion
        )
        install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}-config-version.cmake
                DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}
        )
    endif()

I tried to test this code last week but could not make it work. Another difficulty is that in a static build, the library we install is avif_static, not avif:

    if(BUILD_SHARED_LIBS)
        set(LIBAVIF_INSTALL_TARGET avif)
    else()
        set(LIBAVIF_INSTALL_TARGET avif_static)
    endif()

where avif_static is the merged archive library:

    merge_static_libs(avif_static avif)

I am wondering if avif_static does not inherit the link libraries of avif that we set here.

[vrabaud]

I am playing with an installed static CI right now to debug this.

[wantehchang]

LGTM. Thanks!