Split engine support into a separate internal lib

The chess engine support currently lives in src/, which otherwise
handles only GUI functions. This is a little misplaced, because engines
are not related to the GUI. So lib/ would be a better location for it.

Except all the engine support is actually higher-level than lib/. So we
can nicely fit it in between src/ and lib/ by creating a second internal
lib, libengine.

This is only a small benefit, but it's nice to keep src/ focused on the
GUI.

engine/meson.build

@@ -0,0 +1,33 @@
+libengine_sources = [
+  'ai-profile.vala',
+  'chess-engine-cecp.vala',
+  'chess-engine-uci.vala',
+  'chess-engine.vala',
+]
+
+libengine_vala_args = [
+  '--target-glib=@0@'.format(min_glib_version),
+]
+
+libengine_c_args = [
+  '-include', 'config.h',
+]
+
+libengine_deps = [
+  glib,
+  gio,
+  posix,
+  libchess_dep,
+]
+
+libengine = static_library('libengine', libengine_sources,
+  include_directories: config_h_dir,
+  vala_args: libengine_vala_args,
+  c_args: libengine_c_args,
+  dependencies: libengine_deps
+)
+
+libengine_dep = declare_dependency(
+  link_with: libengine,
+  include_directories: include_directories('.'),
+)

meson.build

@@ -57,6 +57,7 @@ meson.add_install_script('meson_post_install.py')
 subdir('data')
 subdir('po')
 subdir('lib')
+subdir('engine')
 subdir('src')
 subdir('tests')
 subdir('help')

src/meson.build

@@ -8,10 +8,6 @@ resources = gnome.compile_resources(application_id, resource_files,
 # The gnome-chess binary
 chess_sources = [
   resources,
-  'ai-profile.vala',
-  'chess-engine-cecp.vala',
-  'chess-engine-uci.vala',
-  'chess-engine.vala',
   'chess-scene.vala',
   'chess-view.vala',
   'gnome-chess.vala',
@@ -32,6 +28,7 @@ chess_deps = [
   librsvg,
   posix,
   libchess_dep,
+  libengine_dep,
 ]
 
 executable('gnome-chess', chess_sources,