cc-wrapper: deunify clang/gcc treatment of -isystem

In https://github.com/NixOS/nixpkgs/pull/209870 I tried to unify the
treatment of clang and gcc in cc-wrapper as much as possible.
However it appears that I went too far.

Clang requires -isystem flags in order to be able to find gcc's
libstdc++.  Gcc does not need these flags.  If they are added,
gfortran will get confused:

  https://github.com/NixOS/nixpkgs/pull/209870#issuecomment-1500550903

This commit deunifies the chunk of code that adds the -isystem
flags, and explains why this chunk applies only to clang.

pkgs/build-support/cc-wrapper/default.nix

@@ -407,7 +407,11 @@ stdenv.mkDerivation {
       touch "$out/nix-support/libcxx-cxxflags"
       touch "$out/nix-support/libcxx-ldflags"
     ''
-    + optionalString (libcxx == null && (useGccForLibs && gccForLibs.langCC or false)) ''
+    # Adding -isystem flags should be done only for clang; gcc
+    # already knows how to find its own libstdc++, and adding
+    # additional -isystem flags will confuse gfortran (see
+    # https://github.com/NixOS/nixpkgs/pull/209870#issuecomment-1500550903)
+    + optionalString (libcxx == null && isClang && (useGccForLibs && gccForLibs.langCC or false)) ''
       for dir in ${gccForLibs}${lib.optionalString (hostPlatform != targetPlatform) "/${targetPlatform.config}"}/include/c++/*; do
         echo "-isystem $dir" >> $out/nix-support/libcxx-cxxflags
       done