D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082
This will copy the file to the build directory without trying to read
it or substitute values into it.
Also do this optimization if the configuration_data() object passed to
the `configuration:` kwarg is empty, and print a warning about it.
See also: https://github.com/mesonbuild/meson/issues/1542
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.
Jussi Pakkanen
Eli Schwartz
Dylan Baker
Nirbheek Chauhan