diff --git a/lib/libc/aarch64/string/Makefile.inc b/lib/libc/aarch64/string/Makefile.inc
--- a/lib/libc/aarch64/string/Makefile.inc
+++ b/lib/libc/aarch64/string/Makefile.inc
@@ -46,8 +46,8 @@
 # override the generated file in these cases.
 #
 .for FUNC in ${AARCH64_STRING_FUNCS}
-.if !exists(${FUNC}.S)
-${FUNC}.S:
+.if !exists(${LIBC_SRCTOP}/aarch64/string/${FUNC}.S)
+${FUNC}.S: ${LIBC_SRCTOP}/aarch64/string/Makefile.inc
 	printf '/* %sgenerated by libc/aarch64/string/Makefile.inc */\n' @ > ${.TARGET}
 	printf '#define	__%s_aarch64 %s\n' ${FUNC} ${FUNC} >> ${.TARGET}
 	printf '#include "aarch64/%s.S"\n' ${FUNC} >> ${.TARGET}