Factor out the code to find gcc, and add a command-line option to override the

default executable name.
This should help cross-compilation by allowing a cross-gcc to be specified from
the configuration file instead of only through the CC environment variable.

gen/programs.cpp

@@ -0,0 +1,41 @@
+#include "gen/programs.h"
+
+#include "llvm/Support/CommandLine.h"
+#include "llvm/System/Program.h"
+
+#include "root.h"       // error(char*)
+#include "mars.h"       // fatal()
+
+using namespace llvm;
+
+static cl::opt<std::string> gcc("gcc",
+    cl::desc("GCC to use for assembling and linking"),
+    cl::Hidden,
+    cl::ZeroOrMore);
+
+
+sys::Path getGcc() {
+    const char *cc = NULL;
+    
+    if (gcc.getNumOccurrences() > 0 && gcc.length() > 0)
+        cc = gcc.c_str();
+    
+    if (!cc)
+#if !_WIN32
+        cc = getenv("CC");
+    if (!cc)
+#endif
+        cc = "gcc";
+    
+    sys::Path path = sys::Program::FindProgramByName(cc);
+    if (path.empty() && !cc) {
+        if (cc) {
+            path.set(cc);
+        } else {
+            error("failed to locate gcc");
+            fatal();
+        }
+    }
+    
+    return path;
+}