dist: add clang specific -Wno options

Clang 3.1 has more warnings enabled by default than GCC.
Combined with -Werror, they cause the build to fail
unnecessarily.  if the name of our compiler ends in "clang",
add the necessary extra -Wno options.  Ideally we would add
these flags unconditionally, as GCC is supposed to ignore
unknown -Wno flags, but apple's llvm-gcc doesn't.

Fixes #2878.

R=rsc, dave
CC=golang-dev
https://golang.org/cl/5673055

src/cmd/dist/build.c

@@ -582,8 +582,12 @@ install(char *dir)
  splitfields(&gccargs, bstr(&b));
  for(i=0; i<nelem(proto_gccargs); i++)
  vadd(&gccargs, proto_gccargs[i]);
+ if(xstrstr(gccargs.p[0], "clang") != nil) {
+ vadd(&gccargs, "-Wno-dangling-else");
+ vadd(&gccargs, "-Wno-unused-value");
+ }
  }
- 
+
  islib = hasprefix(dir, "lib") || streq(dir, "cmd/cc") || streq(dir, "cmd/gc");
  ispkg = hasprefix(dir, "pkg");
  isgo = ispkg || streq(dir, "cmd/go") || streq(dir, "cmd/cgo");