This adds a GDK_DEBUG=default-settings flag which disables reads
from xsettings and Xft resources, and enables this for the testsuite.
This is one less way to get different testresults depending on the
environment. In particular, it was failing the css tests for me
due to getting the wrong font size because i have a different dpi.
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`.
