assume glibc by default

config.mk

@@ -14,8 +14,11 @@ X11LIB = /usr/X11R6/lib
 INCS = -I. -I/usr/include -I${X11INC}
 LIBS = -L/usr/lib -lc -L${X11LIB} -lX11
 
+# glibc
+CPPFLAGS = -DVERSION=\"${VERSION}\" -D_GNU_SOURCE
+
 # flags
-CPPFLAGS = -DVERSION=\"${VERSION}\"
+#CPPFLAGS = -DVERSION=\"${VERSION}\"
 CFLAGS = -Os ${INCS} ${CPPFLAGS}
 LDFLAGS = -s ${LIBS}
 #CFLAGS = -g -std=c99 -pedantic -Wall -O2 ${INCS} ${CPPFLAGS}