cmake: Look up fish completions with pkg-config

fish-devel ships its desired vendor completions path. Add support for
this to CMakeLists.txt but make it all optional with sane default for
environments that don't want fish in its build deps.

Author: Werkov

scripts/CMakeLists.txt

@@ -1,6 +1,13 @@
 cmake_minimum_required (VERSION 3.22...4.0)
 
-set(FISH_COMPLETIONSDIR share/fish/vendor_completions.d)
+find_package(PkgConfig)
+if(PkgConfig_FOUND)
+	pkg_get_variable(FISH_COMPLETIONSDIR fish completionsdir)
+endif()
+if(NOT FISH_COMPLETIONSDIR)
+	set(FISH_COMPLETIONSDIR share/fish/vendor_completions.d)
+	message(STATUS "fish pkgconfig module missing, assumed completions in ${FISH_COMPLETIONSDIR}")
+endif()
 
 install (DIRECTORY bash vim hooks
          DESTINATION ${TASK_DOCDIR}/scripts)