configure.in: no crypt.h on FreeBSD 12

* configure.in (crypt.h): crypt_r() was added in FreeBSD 12.0 but
  is declared in unistd.h.  [ruby-core:78664] [Bug #13038]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57090 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -2474,6 +2474,8 @@ main(int argc, char **argv)
 ])
 
 AS_IF([test "$ac_cv_func_crypt_r" = yes],
+    [AC_CHECK_HEADERS(crypt.h)])
+AS_IF([test "$ac_cv_func_crypt_r:$ac_cv_header_crypt_h" = yes:yes],
     [AC_CHECK_MEMBERS([struct crypt_data.initialized], [], [],
 		      [AC_INCLUDES_DEFAULT([@%:@include <crypt.h>])])])