krb5: don't include plist support unless CoreFoundation is present

depending what's available when you compile for iOS it's possible to
be __APPLE__ and not have CF; actually test for it instead of blythely
assuming it can be used

cf/framework-corefoundation.m4

@@ -0,0 +1,28 @@
+AC_DEFUN([rk_FRAMEWORK_COREFOUNDATION], [
+
+AC_MSG_CHECKING([for framework CoreFoundation])
+AC_CACHE_VAL(rk_cv_framework_corefoundation,
+[
+if test "$rk_cv_framework_corefoundation" != yes; then
+	ac_save_LIBS="$LIBS"
+	LIBS="$ac_save_LIBS -framework CoreFoundation"
+	AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <CoreFoundation/CoreFoundation.h>
+]],
+[[CFURLRef url;
+char path[] = "/";
+url = CFURLCreateFromFileSystemRepresentation(kCFAllocatorDefault, (UInt8 *)
+path, strlen(path), FALSE);
+CFRelease(url);
+]])],[rk_cv_framework_corefoundation=yes])
+	LIBS="$ac_save_LIBS"
+fi
+])
+
+if test "$rk_cv_framework_corefoundation" = yes; then
+   AC_DEFINE(HAVE_FRAMEWORK_COREFOUNDATION, 1, [Have -framework CoreFoundation])
+   AC_MSG_RESULT(yes)
+else
+   AC_MSG_RESULT(no)
+fi
+AM_CONDITIONAL(FRAMEWORK_COREFOUNDATION, test "$rk_cv_framework_corefoundation" = yes)
+])