git2graph Makefile: be more strict (-Werror) and pass cflags to ld too

tools/git2graph/Makefile

 CC = gcc
 LD = gcc
-CFLAGS = -Wall $(shell pkg-config --cflags libgit2)
+CFLAGS = -Wall -Werror $(shell pkg-config --cflags libgit2)
 LDFLAGS = $(shell pkg-config --libs libgit2)
 
 all: git2graph
 
 git2graph: git2graph.o
-	$(LD) -o $@ $^ $(LDFLAGS)
+	$(LD) -o $@ $^ $(CFLAGS) $(LDFLAGS)
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c $<