The `install` argument for configure_file() was introduced in Meson
0.50, and was ignored in earlier versions.
Since we're still using Meson 0.48 as a baseline, and since it doesn't
cost us nothing to use a conditional in the only place where we used the
`install` argument, let's drop it. This avoids a warning in newer
releases of Meson.
Using an empty `configuration_data` object to copy a configuration file
is deprecated since Meson 0.47 (released July 2018); the equivalent
behaviour is available by using `copy: true`.
