cmd/dist, cmd/go: enable more warnings, make clang errors legible

This does not change the default compiler on OS X to clang.
It appears that for now we can keep using gcc as long as we
enable a few more warning settings that are on-by-default
elsewhere.

R=golang-dev, bradfitz, dave
CC=golang-dev
https://golang.org/cl/11610044

src/cmd/dist/build.c

@@ -407,12 +407,16 @@ static char *proto_gccargs[] = {
 	// native Plan 9 compilers don't like non-standard prototypes
 	// so let gcc catch them.
 	"-Wstrict-prototypes",
+	"-Wextra",
+	"-Wunused",
+	"-Wuninitialized",
 	"-Wno-sign-compare",
 	"-Wno-missing-braces",
 	"-Wno-parentheses",
 	"-Wno-unknown-pragmas",
 	"-Wno-switch",
 	"-Wno-comment",
+	"-Wno-missing-field-initializers",
 	"-Werror",
 	"-fno-common",
 	"-ggdb",
@@ -620,6 +624,8 @@ install(char *dir)
 		for(i=0; i<nelem(proto_gccargs); i++)
 			vadd(&gccargs, proto_gccargs[i]);
 		if(clang) {
+			// disable ASCII art in clang errors, if possible
+			vadd(&gccargs, "-fno-caret-diagnostics");
 			// clang is too smart about unused command-line arguments
 			vadd(&gccargs, "-Qunused-arguments");
 		}

src/cmd/dist/unix.c

@@ -656,6 +656,8 @@ main(int argc, char **argv)
 	setvbuf(stdout, nil, _IOLBF, 0);
 	setvbuf(stderr, nil, _IOLBF, 0);
 
+	setenv("TERM", "dumb", 1); // disable escape codes in clang errors
+
 	binit(&b);
 	
 	slash = "/";

src/cmd/go/build.go

@@ -1811,8 +1811,10 @@ func (b *builder) ccompilerCmd(envvar, defcmd, objdir string) []string {
 		}
 	}
 
-	// clang is too smart about command-line arguments
 	if strings.Contains(a[0], "clang") {
+		// disable ASCII art in clang errors, if possible
+		a = append(a, "-fno-caret-diagnostics")
+		// clang is too smart about command-line arguments
 		a = append(a, "-Qunused-arguments")
 	}
 

src/cmd/go/env.go

@@ -45,6 +45,9 @@ func mkEnv() []envVar {
 		{"GORACE", os.Getenv("GORACE")},
 		{"GOROOT", goroot},
 		{"GOTOOLDIR", toolDir},
+
+		// disable escape codes in clang errors
+		{"TERM", "dumb"},
 	}
 
 	if goos != "plan9" {