[REGRESSION] UnboundLocalError in cmake configure_package_config_file()

[p01arst0rm]

Describe the bug
when specifying install_dir : join_paths(libdir, 'cmake') meson
panics in function configure_package_config_file() ;- line
257 of mesonbuild\modules\cmake.py

To Reproduce

cmake_conf = configuration_data()
cmake_conf.set_quoted('foo', 'bar')

cmake.configure_package_config_file(
    name : 'foolib',
    input : 'foolib.cmake.in',
    install_dir : join_paths(libdir, 'cmake'),
    configuration : cmake_conf)

Expected behavior
generate a configured cmake package config file

system parameters

• native build
• Windows 10 (Current latest patch KB4538461)
• python version 3.7.1
• meson version 0.54.1
• ninja Not Applicable.

[p01arst0rm]

cc @dcbaker :)