Makefile: add DEBUG=true env to enable debug symbols

Author: jiangenj

Makefile

@@ -73,7 +73,14 @@ GITREVDATE=$(shell git log -n 1 --format="%cd" --date=format:%Y%m%d-%H%M%S)
 # Reduces build time and binary sizes considerably.
 # That's only needed if you use gdb or nm.
 # If you need that, build manually without these flags.
-GOFLAGS := "-ldflags=-s -w -X github.com/google/syzkaller/prog.GitRevision=$(REV) -X 'github.com/google/syzkaller/prog.gitRevisionDate=$(GITREVDATE)'"
+LDFLAGS := ""
+DFLAGS := ""
+ifeq ("$(DEBUG)", "true")
+        DFLAGS := -gcflags="all=-N -l"
+else
+	LDFLAGS := -s -w
+endif
+GOFLAGS := "-ldflags=$(LDFLAGS) -X github.com/google/syzkaller/prog.GitRevision=$(REV) -X github.com/google/syzkaller/prog.gitRevisionDate=$(GITREVDATE) $(DFLAGS)"
 ifneq ("$(GOTAGS)", "")
 	GOFLAGS += " -tags=$(GOTAGS)"
 endif