gplugin/gplugin

Make sure gtk4 was found before trying to build vala bindings against it
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10 days ago, Gary Kramlich
7027845d098d
Make sure gtk4 was found before trying to build vala bindings against it

This error only showed up when gtk4 was enabled but not found.

Testing Done:
Built on haiku that doesn't have gtk without setting `-Dgtk4=false`.
Built on debian testing normally.

Reviewed at https://reviews.imfreedom.org/r/3069/
# Dependencies
GPlugin depends on the following at a bare minimum:
* glib-2.0 >= 2.40.0
* gobject-introspection, libgirepository1.0-dev
* meson >= 0.42.0
* gettext
* help2man
* a C compiler
A full build (enabled by default) depends on the following:
* gtk-4
* python3-dev, python-gi-dev, python3-gi
* libperl-dev, libglib-perl, libglib-object-introspection-perl
* liblua5.1-0-dev, lua-lgi
* valac
All of these packages and their development headers need to be installed
prior to building GPlugin.
# Building
GPlugin uses [meson](https://mesonbuild.com/) as its build system. As such
compiling is a little bit different than your typical `./configure`, `make`,
`sudo make install`.
Meson requires you to build in a separate directory than your source. As such,
these instructions use a separate build directory.
To compile you need to run the following commands:
```
meson build
cd build
ninja install
```
If you want/need to tweak the build system (to enable/disable certain loaders)
you can do so at any time by using `meson configure` in the build directory.