Fix installation of bash/fish completions using pkg-config

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

data/meson.build

@@ -70,6 +70,20 @@ fish_completion = configure_file(
   configuration: conf_data
 )
 
-install_data(bash_completion, install_dir: join_paths(datadir, 'bash-completion', 'completions'))
+bash_comp = dependency('bash-completion', required: false)
+if bash_comp.found()
+  bash_compdir = bash_comp.get_pkgconfig_variable('completionsdir')
+else
+  bash_compdir = join_paths(datadir, 'bash-completion', 'completions')
+endif
+
+fish_comp = dependency('fish', required: false)
+if fish_comp.found()
+  fish_compdir = fish_comp.get_pkgconfig_variable('completionsdir')
+else
+  fish_compdir = join_paths(datadir, 'fish', 'completions')
+endif
+
+install_data(bash_completion, install_dir: bash_compdir)
 install_data(zsh_completion, install_dir: join_paths(datadir, 'zsh', 'site-functions'))
-install_data(fish_completion, install_dir: join_paths(datadir, 'fish', 'completions'))
+install_data(fish_completion, install_dir: fish_compdir)