enable stack protector and compile to position independent executable

config.mk

@@ -11,7 +11,7 @@ LIBS = -L/usr/lib -lc -L${X11LIB} -lX11
 
 CPPFLAGS = -D_GNU_SOURCE
 # -Wno-unused-function for routines not activated by user
-CFLAGS = -std=c99 -pedantic -Wno-unused-function -Wall -Wextra -Os ${INCS} ${CPPFLAGS}
+CFLAGS = -std=c99 -pedantic -Wno-unused-function -Wall -Wextra -Os -fstack-protector-strong -fstack-check -fPIE ${INCS} ${CPPFLAGS}
 LDFLAGS = ${LIBS}
 
 CC = cc