The configure_file command raised an exception when an input was specified as a
File, because os.path.join does not take File objects directly. This patch
converts a File object to a string and adjusts the subsequent os.path.join
calls.
The same substitutions and rules as custom_target().
Also generally fix it to actually work when run in a subdir and with
anything other than absolute paths for input and output files.
We now also log a message when configuring files.
Includes tests for all this.
Add support for passing a description to configuration data
setter methods via a 'description' kwarg. The description
string will be used when meson generates the entire configure
file without a template, autoconf-style.