Set ac_cv_strftime_c99_support to empty string on cross-compile

Author: zanieb

configure

@@ -6686,7 +6686,7 @@ int main(void)
 ]])],
 [ac_cv_strftime_c99_support=yes],
 [AC_MSG_ERROR([Python requires C99-compatible strftime specifiers])],
-[])])
+[ac_cv_strftime_c99_support=])])
 
 dnl check for ncursesw/ncurses and panelw/panel
 dnl NOTE: old curses is not detected.