Fix crypt_r detection under BSD system

For BSD systems, crypt.h does not exist, instead we need to
include unistd.h.

Closes GH-5971.

Author: devnexen

build/php.m4

@@ -2217,6 +2217,17 @@ crypt_r("passwd", "hash", &buffer);
     fi
     ])
 
+    if test "$php_cv_crypt_r_style" = "none"; then
+      AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+#include <stdlib.h>
+#include <unistd.h>
+]],[[
+struct crypt_data buffer;
+crypt_r("passwd", "hash", &buffer);
+]])],[php_cv_crypt_r_style=struct_crypt_data],[])
+    fi
+    ])
+
   if test "$php_cv_crypt_r_style" = "cryptd"; then
     AC_DEFINE(CRYPT_R_CRYPTD, 1, [Define if crypt_r has uses CRYPTD])
   fi