completion: use pkg-config to get install location for bash/fish

Both shells provide pkg-config files which declare their designated
completionsdir. Use this as the primary source of truth.

meson.build

@@ -40,6 +40,8 @@ wayland_protos = dependency('wayland-protocols', version: '>=1.14')
 xkbcommon      = dependency('xkbcommon')
 cairo          = dependency('cairo')
 gdk_pixbuf     = dependency('gdk-pixbuf-2.0', required: get_option('gdk-pixbuf'))
+bash_comp      = dependency('bash-completion', required: false)
+fish_comp      = dependency('fish', required: false)
 libpam         = cc.find_library('pam', required: get_option('pam'))
 crypt          = cc.find_library('crypt', required: not libpam.found())
 math           = cc.find_library('m')
@@ -196,7 +198,11 @@ if get_option('bash-completions')
 	bash_files = files(
 		'completions/bash/swaylock',
 	)
-	bash_install_dir = datadir + '/bash-completion/completions'
+	if bash_comp.found()
+		bash_install_dir = bash_comp.get_pkgconfig_variable('completionsdir')
+	else
+		bash_install_dir = datadir + '/bash-completion/completions'
+	endif
 
 	install_data(bash_files, install_dir: bash_install_dir)
 endif
@@ -205,7 +211,11 @@ if get_option('fish-completions')
 	fish_files = files(
 		'completions/fish/swaylock.fish',
 	)
-	fish_install_dir = datadir + '/fish/completions'
+	if fish_comp.found()
+		fish_install_dir = fish_comp.get_pkgconfig_variable('completionsdir')
+	else
+		fish_install_dir = datadir + '/fish/completions'
+	endif
 
 	install_data(fish_files, install_dir: fish_install_dir)
 endif