The dependency on libXRes should consider the minimum version

Hi Oliver, I'm backporting from xfce 4.16 to debian stretch. 😬 Until now everything compiles fine, except that xfce4 depends on libXRes without controlling version. and fail due old version and missing some definitions.

hints.c: In function 'getWindowPID':
hints.c:1465:5: error: unknown type name 'XResClientIdSpec'
     XResClientIdSpec client_specs;
     ^~~~~~~~~~~~~~~~
hints.c:1466:5: error: unknown type name 'XResClientIdValue'
     XResClientIdValue *client_ids = NULL;
     ^~~~~~~~~~~~~~~~~
hints.c:1473:21: error: request for member 'client' in something not a structure or union
         client_specs.client = w;
                     ^
hints.c:1474:21: error: request for member 'mask' in something not a structure or union
         client_specs.mask = XRES_CLIENT_ID_PID_MASK;
                     ^
hints.c:1474:29: error: 'XRES_CLIENT_ID_PID_MASK' undeclared (first use in this function)
         client_specs.mask = XRES_CLIENT_ID_PID_MASK;
                             ^~~~~~~~~~~~~~~~~~~~~~~
hints.c:1474:29: note: each undeclared identifier is reported only once for each function it appears in
hints.c:1478:9: warning: implicit declaration of function 'XResQueryClientIds' [-Wimplicit-function-declaration]
         XResQueryClientIds (display_info->dpy, 1, &client_specs, &num_ids, &client_ids);
         ^~~~~~~~~~~~~~~~~~
hints.c:1486:34: error: request for member 'spec' in something not a structure or union
                 if (client_ids[i].spec.mask == XRES_CLIENT_ID_PID_MASK)
                                  ^
hints.c:1488:50: error: request for member 'value' in something not a structure or union
                     CARD32 *value = client_ids[i].value;
                                                  ^
Makefile:816: recipe for target 'xfwm4-hints.o' failed

XResClientIdSpec was defined on libXRes 1.2.0 and debian stretch has just 1.0.7.

I'll probably do a patch soon but first wanted to document it

Thanks.