Big rewrite / improvement of target handling.

If no CPU is specified, use the host CPU type, not just a default of "nehalem".
Provide better features strings to the LLVM target machinery.
 -> Thus ensuring that LLVM doesn't generate SSE>2 instructions for the SSE2
    target (Fixes issue #82).
 -> Slight code improvements from using cmovs in generated code now
Use the llvm popcnt intrinsic for the SSE2 target now (it now generates code
  that doesn't call the popcnt instruction now that we properly tell LLVM
  which instructions are and aren't available for SSE2.)

ispc_test.cpp

@@ -171,7 +171,21 @@ static bool lRunTest(const char *fn) {
     }
 
     std::string eeError;
+#if defined(LLVM_3_0) || defined(LLVM_3_0svn)
+    llvm::EngineBuilder engineBuilder(module);
+    engineBuilder.setErrorStr(&eeError);
+    engineBuilder.setEngineKind(llvm::EngineKind::JIT);
+#if 0
+    std::vector<std::string> attributes;
+    if (target != NULL && !strcmp(target, "avx"))
+        attributes.push_back("+avx");
+    engineBuilder.setMAttrs(attributes);
+    engineBuilder.setUseMCJIT(true);
+#endif
+    llvm::ExecutionEngine *ee = engineBuilder.create();
+#else
     llvm::ExecutionEngine *ee = llvm::ExecutionEngine::createJIT(module, &eeError);
+#endif
     if (!ee) {
         fprintf(stderr, "Unable to create ExecutionEngine: %s\n", eeError.c_str());
         return false;
@@ -338,6 +352,11 @@ static bool lRunTest(const char *fn) {
 
 int main(int argc, char *argv[]) {
     llvm::InitializeNativeTarget();
+#if defined(LLVM_3_0) || defined(LLVM_3_0svn)
+    llvm::InitializeAllTargetMCs();
+    LLVMLinkInMCJIT();
+    LLVMLinkInJIT();
+#endif
 
     std::vector<const char *> files;
     for (int i = 1; i < argc; ++i) {