config.mk: use -std=c99

config.mk

@@ -24,7 +24,7 @@ LIBS = -L${X11LIB} -lX11 ${XINERAMALIBS} ${FREETYPELIBS}
 
 # flags
 CPPFLAGS = -D_BSD_SOURCE -D_XOPEN_SOURCE=700 -D_POSIX_C_SOURCE=200809L -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
-CFLAGS   = -ansi -pedantic -Wall -Os ${INCS} ${CPPFLAGS}
+CFLAGS   = -std=c99 -pedantic -Wall -Os ${INCS} ${CPPFLAGS}
 LDFLAGS  = -s ${LIBS}
 
 # compiler and linker