agda.withPackages: allow installing Agda without GHC using ghc = null;

doc/languages-frameworks/agda.section.md

@@ -121,6 +121,8 @@ agda.withPackages {
 }
 ```
 
+To install Agda without GHC, use `ghc = null;`.
+
 ## Writing Agda packages {#writing-agda-packages}
 
 To write a nix derivation for an Agda library, first check that the library has a `*.agda-lib` file.

pkgs/build-support/agda/default.nix

@@ -66,7 +66,7 @@ let
       ''
         mkdir -p $out/bin
         makeWrapper ${Agda.bin}/bin/agda $out/bin/agda \
-          --add-flags "--with-compiler=${ghc}/bin/ghc" \
+          ${lib.optionalString (ghc != null) ''--add-flags "--with-compiler=${ghc}/bin/ghc"''} \
           --add-flags "--library-file=${library-file}"
         ln -s ${Agda.bin}/bin/agda-mode $out/bin/agda-mode
       '';