configure: don't check for Security.framework when cross-compiling

Since it checks for the local file, not the cross-compiled one.

Closes #5189
diff --git a/configure.ac b/configure.ac
index 889617f..0479099 100755
--- a/configure.ac
+++ b/configure.ac
@@ -1554,7 +1554,7 @@
 AC_MSG_CHECKING([whether to enable Secure Transport])
 if test -z "$ssl_backends" -o "x$OPT_SECURETRANSPORT" != xno; then
   if test "x$OPT_SECURETRANSPORT" != "xno" &&
-     test -d "/System/Library/Frameworks/Security.framework"; then
+     (test "x$cross_compiling" != "xno" || test -d "/System/Library/Frameworks/Security.framework"); then
     AC_MSG_RESULT(yes)
     AC_DEFINE(USE_SECTRANSP, 1, [enable Secure Transport])
     AC_SUBST(USE_SECTRANSP, [1])