Split meson up to be more modular

This also explicitly constructs reusable library components to make it
easier to extend the project over time. Notably we make less assumptions
about the host system and use pkgconfig where appropriate to give us
system details, such as the systemd system unit directory for packaging.
This can be overriden with the meson option.

Signed-off-by: Ikey Doherty <ikey@solus-project.com>

example/meson.build

@@ -0,0 +1,11 @@
+# An example game
+executable(
+    'example',
+    sources: [
+        'main.c',
+    ],
+    include_directories: libgamemode_includes,
+    dependencies: [
+        libdl,
+    ],
+)