Don't generate colorized output escapes when stderr isn't a TTY.

When piping to a pile, more/less, etc, this is generally undesirable.

This behavior can be overridden with the --colorized-output command-line
flag.

ispc.cpp

@@ -578,6 +578,7 @@ Globals::Globals() {
     disableWarnings = false;
     warningsAsErrors = false;
     quiet = false;
+    forceColoredOutput = false;
     disableLineWrap = false;
     emitPerfWarnings = true;
     emitInstrumentation = false;

ispc.h

@@ -407,6 +407,10 @@ struct Globals {
     /** Indicates whether all printed output should be surpressed. */
     bool quiet;
 
+    /** Always use ANSI escape sequences to colorize warning and error
+        messages, even if piping output to a file, etc. */
+    bool forceColoredOutput;
+
     /** Indicates whether calls should be emitted in the program to an
         externally-defined program instrumentation function. (See the
         "Instrumenting your ispc programs" section in the user's

main.cpp

@@ -83,6 +83,9 @@ usage(int ret) {
     printf("    [--arch={%s}]\t\tSelect target architecture\n", 
            Target::SupportedTargetArchs());
     printf("    [--c++-include-file=<name>]\t\tSpecify name of file to emit in #include statement in generated C++ code.\n");
+#ifndef ISPC_IS_WINDOWS
+    printf("    [--colored-output]\t\tAlways use terminal colors in error/warning messages.\n");
+#endif
     printf("    [--cpu=<cpu>]\t\t\tSelect target CPU type\n");
     printf("         <cpu>={%s}\n", Target::SupportedTargetCPUs().c_str());
     printf("    [-D<foo>]\t\t\t\t#define given value when running preprocessor\n");
@@ -401,6 +404,8 @@ int main(int Argc, char *Argv[]) {
 #ifndef ISPC_IS_WINDOWS
         else if (!strcmp(argv[i], "--pic"))
             generatePIC = true;
+        else if (!strcmp(argv[i], "--colored-output"))
+            g->forceColoredOutput = true;
 #endif // !ISPC_IS_WINDOWS
         else if (!strcmp(argv[i], "--quiet"))
             g->quiet = true;

util.cpp

@@ -44,6 +44,7 @@
 #endif
 #else
 #include <alloca.h>
+#include <unistd.h>
 #endif
 #include <stdio.h>
 
@@ -94,6 +95,10 @@ static bool
 lHaveANSIColors() {
     static bool r = (getenv("TERM") != NULL &&
                      strcmp(getenv("TERM"), "dumb") != 0);
+#ifndef ISPC_IS_WINDOWS
+    r &= isatty(2);
+#endif // !ISPC_IS_WINDOWS
+    r |= g->forceColoredOutput;
     return r;
 }