libconfig: Update to 1.7.3

libconfig: Update to 1.7.3

...it just came my way...

For details see:
https://github.com/hyperrealm/libconfig/releases/tag/v1.7.3

This release includes some bugfixes and enhancements.

    Fixed a memory access violation bug in config_clear()
    Various fixes to CMake and Visual Studio build files
    Added a Setting::isString() method
    Fixed a bug in config_setting_lookup() where the setting itself
    (instead of NULL) would be returned if the path was not found.
    Renamed all remaining internal methods that lacked a 'libconfig_'
    prefix.

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>