haskellPackages.mkDerivation: avoid forcing stdenv.cc if missing

If we don't check `hasCC`, we run into trouble on pkgsCross.ghcjs, one
of the few platforms where `cc` will throw
(so `stdenv.cc.isClang or false would` not be enough).

Problem introduced in 8164b190 / #266172.

pkgs/development/haskell-modules/generic-builder.nix

···
 
 # Implicit pointer to integer conversions are errors by default since clang 15.
 # Works around https://gitlab.haskell.org/ghc/ghc/-/issues/23456.
-// lib.optionalAttrs stdenv.cc.isClang {
+// lib.optionalAttrs (stdenv.hasCC && stdenv.cc.isClang) {
   NIX_CFLAGS_COMPILE = "-Wno-error=int-conversion";
 }
 
 # Ensure libc++abi is linked even when clang is invoked as just `clang` or `cc`.
 # Works around https://github.com/NixOS/nixpkgs/issues/166205.
 # This can be dropped once a fix has been committed to cc-wrapper.
-// lib.optionalAttrs (stdenv.cc.isClang && stdenv.cc.libcxx != null) {
+// lib.optionalAttrs (stdenv.hasCC && stdenv.cc.isClang && stdenv.cc.libcxx != null) {
   NIX_LDFLAGS = "-l${stdenv.cc.libcxx.cxxabi.libName}";
 }
 )