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.)

Makefile

@@ -10,7 +10,12 @@ CLANG_LIBS = -lclangFrontend -lclangDriver \
              -lclangSerialization -lclangParse -lclangSema \
              -lclangAnalysis -lclangAST -lclangLex -lclangBasic
 
-LLVM_LIBS=$(shell llvm-config --ldflags --libs) -lpthread -ldl
+ISPC_LIBS=$(CLANG_LIBS) \
+	$(shell llvm-config --ldflags --libs backend bitreader bitwriter codegen engine mcjit scalaropts native analysis core instcombine ipa ipo linker instrumentation) \
+	-lpthread -ldl
+ISPC_TEST_LIBS=$(shell llvm-config --ldflags --libs bitreader backend interpreter engine jit mcjit) \
+	-lpthread -ldl
+
 LLVM_CXXFLAGS=$(shell llvm-config --cppflags)
 LLVM_VERSION=$(shell llvm-config --version | sed s/\\./_/)
 LLVM_VERSION_DEF=-DLLVM_$(LLVM_VERSION)
@@ -80,11 +85,11 @@ doxygen:
 
 ispc: print_llvm_src dirs $(OBJS)
 	@echo Creating ispc executable
-	@$(CXX) $(LDFLAGS) -o $@ $(OBJS) $(CLANG_LIBS) $(LLVM_LIBS)
+	@$(CXX) $(LDFLAGS) -o $@ $(OBJS) $(ISPC_LIBS)
 
 ispc_test: dirs ispc_test.cpp
 	@echo Creating ispc_test executable
-	@$(CXX) $(LDFLAGS) $(CXXFLAGS) -o $@ ispc_test.cpp $(LLVM_LIBS)
+	@$(CXX) $(LDFLAGS) $(CXXFLAGS) -o $@ ispc_test.cpp $(ISPC_TEST_LIBS)
 
 objs/%.o: %.cpp
 	@echo Compiling $<