Cannot go to a new line in the middle of an if.

The `install` argument for configure_file() is not needed if set to a
`true` constant value while passing `install_dir`.

Let's use what Debian stable provides.

Use non-deprecated syntax to generate the main pkg-config file; then,
generate all the backend-specific pkg-config files by simply depending
on clutter-1.0.

Don't use something from ancient times, even if we haven't touched
Clutter in a long while.

Older versions of Meson do not deal with concatenation between mixed
values very well.

Build fixes

See merge request GNOME/clutter!13

The `leave` handler checks for NULL surfaces, but the `enter` handler
does not.

Fixes: #17

wayland: Protect against enter events for empty surfaces

Closes #17

See merge request GNOME/clutter!14

https://bugzilla.gnome.org/show_bug.cgi?id=783791

https://bugzilla.gnome.org/show_bug.cgi?id=783791

wayland: Fix incorrect button mask in clutter_wayland_handle_button

See merge request GNOME/clutter!15

Fix a typo.

For builds using the Visual Studio headers, we must define
_USE_MATH_DEFINES in order for _M_PI to work.

We forgot to put into clutter-config.h the Windows items, which means
that we would actually build the Win32 parts in vain.

We may not be running the tests on an X11-based system

...on Visual Studio builds.  We can get rid of lots of compiler noise
with that header, which we are free to use since we are always using
GLib.

Also, define _GNU_SOURCE when we are not using Visual Studio.

Eliminate warning C4098 (returning value in a void function), which
is considered an error if building against GLib-2.68.x or later.

Fix some Windows items

See merge request GNOME/clutter!16