Add support for -I command-line argument to specify #include search directories.

ispc.h

@@ -423,6 +423,10 @@ struct Globals {
     /** Arguments to pass along to the C pre-processor, if it is run on the
         program before compilation. */
     std::vector<std::string> cppArgs;
+
+    /** Additional user-provided directories to search when processing
+        #include directives in the preprocessor. */
+    std::vector<std::string> includePath;
 };
 
 enum {

main.cpp

@@ -103,6 +103,7 @@ usage(int ret) {
     printf("    [--help]\t\t\t\tPrint help\n");
     printf("    [--help-dev]\t\t\tPrint help for developer options\n");
     printf("    [-h <name>/--header-outfile=<name>]\tOutput filename for header\n");
+    printf("    [-I <path>]\t\t\t\tAdd <path> to #include file search path\n");
     printf("    [--instrument]\t\t\tEmit instrumentation to gather performance data\n");
     printf("    [--math-lib=<option>]\t\tSelect math library\n");
     printf("        default\t\t\t\tUse ispc's built-in math functions\n");
@@ -287,6 +288,15 @@ int main(int Argc, char *Argv[]) {
             ot = Module::Bitcode;
         else if (!strcmp(argv[i], "--emit-obj"))
             ot = Module::Object;
+        else if (!strcmp(argv[i], "-I")) {
+            if (++i == argc) {
+                fprintf(stderr, "No path specified after -I option.\n");
+                usage(1);
+            }
+            g->includePath.push_back(argv[i]);
+        }
+        else if (!strncmp(argv[i], "-I", 2))
+            g->includePath.push_back(argv[i]+2);
         else if (!strcmp(argv[i], "--fuzz-test"))
             g->enableFuzzTest = true;
         else if (!strncmp(argv[i], "--fuzz-seed=", 12))

module.cpp

@@ -1175,6 +1175,20 @@ Module::execPreprocessor(const char* infilename, llvm::raw_string_ostream* ostre
     // track the source file position by handling them ourselves.
     inst.getPreprocessorOutputOpts().ShowComments = 1;
 
+    clang::HeaderSearchOptions &headerOpts = inst.getHeaderSearchOpts();
+    headerOpts.UseBuiltinIncludes = 0;
+#ifndef LLVM_2_9
+    headerOpts.UseStandardSystemIncludes = 0;
+#endif // !LLVM_2_9
+    headerOpts.UseStandardCXXIncludes = 0;
+    if (g->debugPrint)
+        headerOpts.Verbose = 1;
+    for (int i = 0; i < (int)g->includePath.size(); ++i)
+        headerOpts.AddPath(g->includePath[i], clang::frontend::Angled,
+                           true /* is user supplied */,
+                           false /* not a framework */,
+                           true /* ignore sys root */);
+
     clang::PreprocessorOptions &opts = inst.getPreprocessorOpts();
 
     // Add defs for ISPC and PI