diff --git a/config.m4 b/config.m4
index f1fcb05..d73dcb7 100644
--- a/config.m4
+++ b/config.m4
@@ -12,7 +12,7 @@ if test "$PHP_XPASS" != "no"; then
   PHP_EVAL_LIBLINE([$LIBXCRYPT_LIBS], [XPASS_SHARED_LIBADD])
 
   old_CFLAGS=$CFLAGS; CFLAGS="$CFLAGS $LIBXCRYPT_CFLAGS"
-  old_LDFLAGS=$LDFLAGS; LDFLAGS="$LIBXCRYPT_LIBS $LDFLAGS"
+  old_LIBS=$LIBS; LIBS="$LIBS $LIBXCRYPT_LIBS"
 
   AC_MSG_CHECKING([for yescrypt])
   AC_RUN_IFELSE([AC_LANG_SOURCE([[
@@ -20,7 +20,6 @@ if test "$PHP_XPASS" != "no"; then
 #include <unistd.h>
 #include <crypt.h>
 #include <stdlib.h>
-#include <string.h>
 
 int main(void) {
     char salt[8];
@@ -39,7 +38,6 @@ int main(void) {
 #include <unistd.h>
 #include <crypt.h>
 #include <stdlib.h>
-#include <string.h>
 
 int main(void) {
     char salt[8];
@@ -53,7 +51,7 @@ int main(void) {
   ])
 
   CFLAGS=$old_CFLAGS
-  LDFLAGS=$old_LDFLAGS
+  LIBS=$old_LIBS
 
   PHP_SUBST([XPASS_SHARED_LIBADD])