Try to include the same things and in a similar order so differences
are easier to catch.
This also adds the backend specific .pc files for gdk like gdk-x11-3.0.pc
Build the input modules for GTK+, either as modules or built directly
into GTK. Also provide a configure option to build the specified
immodules, or all, or the backend immodule(s) or none of the immodules
into GTK. Note that for Visual Studio all immodules are built into
the GTK DLL by default, like what is done in the Visual Studio projects.
Note that building the backend immodules for Quartz, X11 and Wayland are
currently untested.
This is so that the post install script will work on environments where
*NIX shell scripts are not supported, such as on Windows cmd.exe for
Visual Studio builds.
PangoFT2 is optional on Windows, so we only really need the fallback if
when it is required.
Along with that, since FreeType does not typically ship with pkg-config
files in its CMake builds, check for the needed headers, .lib and
function and then use the fallback when they could not be found and
PangoFT2 is used.
On Visual Studio builds, since Cairo builds tend not to generate
pkg-config files for us, look for the headers and .lib's, before
attempting to download the Cairo repo (which is quite large) and
building it.
We can simplify this process when Meson gains the ability to check
for the dependencies in a declarative fashion, but before that, this
is what must be done.
