treewide: make -Wno-error flags clang-only

In 724e833ea2, I was a little too aggressive in enabling these flags.
Many don’t work in gcc, and we should probably avoid settings them
widely. This makes those flags optional on isclang

pkgs/applications/science/astronomy/xplanet/default.nix

@@ -17,7 +17,7 @@ stdenv.mkDerivation rec {
     ./gcc6.patch
   ];
 
-  NIX_CFLAGS_COMPILE = "-Wno-error=c++11-narrowing";
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=c++11-narrowing";
 
   meta = {
     description = "Renders an image of the earth or other planets into the X root window";

pkgs/applications/science/biology/cmtk/default.nix

@@ -11,7 +11,7 @@ stdenv.mkDerivation rec {
 
   buildInputs = [cmake];
 
-  NIX_CFLAGS_COMPILE = "-Wno-error=c++11-narrowing";
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=c++11-narrowing";
 
   meta = with stdenv.lib; {
     description     = "Computational Morphometry Toolkit ";

pkgs/development/libraries/bullet/default.nix

@@ -40,7 +40,7 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
-  NIX_CFLAGS_COMPILE = "-Wno-error=argument-outside-range";
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=argument-outside-range";
 
   meta = with stdenv.lib; {
     description = "A professional free 3D Game Multiphysics Library";

pkgs/development/libraries/clucene-core/2.x.nix

@@ -36,7 +36,7 @@ stdenv.mkDerivation rec {
   # /build/clucene-core-2.3.3.4/build/bin/cl_test"
   doCheck = false;
 
-  NIX_CFLAGS_COMPILE = "-Wno-error=c++11-narrowing";
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=c++11-narrowing";
 
   meta = with stdenv.lib; {
     description = "Core library for full-featured text search engine";

pkgs/tools/archivers/p7zip/default.nix

@@ -37,7 +37,7 @@ stdenv.mkDerivation rec {
 
   setupHook = ./setup-hook.sh;
 
-  NIX_CFLAGS_COMPILE = "-Wno-error=c++11-narrowing";
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=c++11-narrowing";
 
   meta = {
     homepage = http://p7zip.sourceforge.net/;

pkgs/tools/compression/pbzip2/default.nix

@@ -17,7 +17,7 @@ stdenv.mkDerivation rec {
 
   installFlags = "PREFIX=$(out)";
 
-  NIX_CFLAGS_COMPILE = "-Wno-error=reserved-user-defined-literal";
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=reserved-user-defined-literal";
 
   meta = with stdenv.lib; {
     homepage = http://compression.ca/pbzip2/;

pkgs/tools/networking/ntopng/default.nix

@@ -53,7 +53,8 @@ stdenv.mkDerivation rec {
     sed 's|LIBS += -lstdc++.6||' -i Makefile
   '';
 
-  NIX_CFLAGS_COMPILE = [ "-fpermissive" "-Wno-error=reserved-user-defined-literal" ];
+  NIX_CFLAGS_COMPILE = [ "-fpermissive" ]
+    ++ stdenv.lib.optional stdenv.cc.isClang "-Wno-error=reserved-user-defined-literal";
 
   meta = with stdenv.lib; {
     description = "High-speed web-based traffic analysis and flow collection tool";