rtmpdump: fix cross with Clang

"clang" is not the correct value for CC when cross compiling — it
should be the prefixed, wrapped clang for the host platform.  Let's
force the use of our original CC in all cases.

pkgs/tools/video/rtmpdump/default.nix

@@ -31,14 +31,17 @@ stdenv.mkDerivation {
     })
   ];
 
+  preBuild = ''
+    makeFlagsArray+=(CC="$CC")
+  '';
+
   makeFlags = [
     "prefix=$(out)"
     "CROSS_COMPILE=${stdenv.cc.targetPrefix}"
   ]
     ++ lib.optional gnutlsSupport "CRYPTO=GNUTLS"
     ++ lib.optional opensslSupport "CRYPTO=OPENSSL"
-    ++ lib.optional stdenv.hostPlatform.isDarwin "SYS=darwin"
-    ++ lib.optional stdenv.cc.isClang "CC=clang";
+    ++ lib.optional stdenv.hostPlatform.isDarwin "SYS=darwin";
 
   propagatedBuildInputs = [ zlib ]
     ++ lib.optionals gnutlsSupport [ gnutls nettle ]