Adding debug and clang targets, changing asan target

Makefile

@@ -134,12 +134,18 @@ ispc: print_llvm_src dirs $(OBJS)
 	@echo Creating ispc executable
 	@$(CXX) $(OPT) $(LDFLAGS) -o $@ $(OBJS) $(ISPC_LIBS)
 
+# Use clang as a default compiler, instead of gcc
+clang: ispc
+clang: CXX=clang++
+
 # Build ispc with address sanitizer instrumentation using clang compiler
 # Note that this is not portable build
-asan: ispc
-asan: CXX=clang++
+asan: clang
 asan: OPT+=-fsanitize=address
 
+# Do debug build, i.e. -O0 -g
+debug: ispc
+debug: OPT=-O0 -g
 
 objs/%.o: %.cpp
 	@echo Compiling $<