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`.
This is not much of a test, tbh. But it will give us a convenient
dump of all GTK+ settings in continuous, which helps with tracking
down test failures that might depend on settings.
Makefile.decl does not work well with scripts that stay
in srcdir, so generate test-simplify from test-simplify.in,
just os that it ends up in builddir.
