build: make completions respect install prefixes

Tell pkgconfig about prefix and datadir as required in the .pc files, so
if the prefix isn't standard nothing is installed outside of it.

For fish, this requires https://github.com/fish-shell/fish-shell/pull/6778

Fixes swaywm/swaybg#13

meson.build

@@ -263,7 +263,10 @@ if get_option('bash-completions')
 		'completions/bash/swaymsg',
 	)
 	if bash_comp.found()
-		bash_install_dir = bash_comp.get_pkgconfig_variable('completionsdir')
+		bash_install_dir = bash_comp.get_pkgconfig_variable(
+			'completionsdir',
+			define_variable: ['datadir', datadir]
+		)
 	else
 		bash_install_dir = join_paths(datadir, 'bash-completion', 'completions')
 	endif
@@ -278,7 +281,10 @@ if get_option('fish-completions')
 		'completions/fish/swaynag.fish',
 	)
 	if fish_comp.found()
-		fish_install_dir = fish_comp.get_pkgconfig_variable('completionsdir')
+		fish_install_dir = fish_comp.get_pkgconfig_variable(
+			'completionsdir',
+			define_variable: ['datadir', datadir]
+		)
 	else
 		fish_install_dir = join_paths(datadir, 'fish', 'vendor_completions.d')
 	endif