FindXMLRPC.cmake fails if "xmlrpc-c-config <modules> --cflags" outputs nothing to stdout
This is exactly what happens with xmlrpc-c-devel on Fedora 26 since upgrade from 1.48.0-4.fc26 to 1.49.02-2.fc26. In the new version "/usr/bin/xmlrpc-c-config c++2 --cflags" returns empty rather than "-I/usr/include/". Since /usr/include is in the default include path it seems reasonable enough to exclude it. Since FindXMLRPC.cmake already checks the exit code from "xmlrpc-c-config --cflags" the fix is simply to drop the requirement that XMLRPC_INCLUDE_DIRS is non-empty.
The bug that triggered the update to xmlrpc-c on Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=1482842
Steps to reproduce: Run cmake (e.g version 3.9.1) on a Fedora 26 machine with xmlrpc 1.49.02-2.fc26 on a project with the following CMakeLists.txt:
cmake_minimum_required(VERSION 3.2.0) find_package(XMLRPC REQUIRED COMPONENTS c++2 client)