Configurator: try to call pkgconf before failing back to pkg-config

Pkgconf is a modern implementation of the "pkg-config freedesktop standard". Linux distros actually use pkgconf as their
pkg-config implementation and provide a in place replacement. (Homebrew packages both and is the last place I found where you can get the historical implementation).
It is always OK to call pkgconf for pkg-config all the CLI and outputs within the standard are following the standard :-).

Cygwin pkg-config package though provides pkgconf but does not provide the alias pkg-config forcing us to uptream the switch to the tool name instead of the standard.

Signed-off-by: Pierre Boutillier <pierre.boutillier@laposte.net>

otherlibs/configurator/src/v1.ml

@@ -658,18 +658,21 @@ module Pkg_config = struct
     }
 
   let get c =
-    let pkg_config_exe_name =
-      match Sys.getenv "PKG_CONFIG" with
-      | s -> s
-      | exception Not_found -> "pkg-config"
-    in
     let pkg_config_args =
       match Sys.getenv "PKG_CONFIG_ARGN" with
       | s -> String.split ~on:' ' s
       | exception Not_found -> []
     in
-    Option.map (which c pkg_config_exe_name) ~f:(fun pkg_config ->
-      { pkg_config; pkg_config_args; configurator = c })
+    match Sys.getenv "PKG_CONFIG" with
+    | s ->
+      Option.map (which c s) ~f:(fun pkg_config ->
+        { pkg_config; pkg_config_args; configurator = c })
+    | exception Not_found ->
+      (match which c "pkgconf" with
+       | None ->
+         Option.map (which c "pkg-config") ~f:(fun pkg_config ->
+           { pkg_config; pkg_config_args; configurator = c })
+       | Some pkg_config -> Some { pkg_config; pkg_config_args; configurator = c })
   ;;
 
   type package_conf =