disable LTO for easier platform compatibility
honor CFLAGS

Index: makefile
--- makefile.orig
+++ makefile
@@ -1,2 +1,2 @@
 highlight-pointer: highlight-pointer.c
-	$(CC) $^ -o $@ -flto -O3 -Wall -Wextra -Wshadow -std=c99 -lX11 -lXext -lXfixes -lXi
+	$(CC) $^ -o $@ $(CFLAGS) -Wextra -Wshadow -std=c99 -lX11 -lXext -lXfixes -lXi