erlang: Generalize Erlang/OTP derivations.

Switch official Erlang versions to use a common builder.

pkgs/development/beam-modules/lib.nix

@@ -0,0 +1,21 @@
+{ pkgs }:
+
+rec {
+
+  /* Uses generic-builder to evaluate provided drv containing OTP-version specific data.
+
+  drv: package containing version-specific args;
+  builder: generic builder for all Erlang versions;
+  gargs: arguments passed to the generic-builder, used mostly to customize dependencies;
+  args: arguments merged into version-specific args, used mostly to enable/disable high-level OTP
+    features, like ODBC or WX support;
+
+  Please note that "mkDerivation" defined here is the one called from R16.nix and similar files.
+  */
+  callErlang = drv: gargs: args: pkgs.callPackage drv (
+    let builder = pkgs.callPackage ../../development/interpreters/erlang/generic-builder.nix gargs;
+    in {
+      mkDerivation = a: builder (a // args);
+    });
+
+}