Extend X11 input driver with XInput2 extensions for multi-mouse

Add multi-mouse support for X11, by using XInput2 extensions. Only
master pointers are considered as mice.

New behavior put behind HAVE_XI2 and configure script extended with
necessary detection.

Author: zoltanvb

Makefile.common

@@ -1350,6 +1350,10 @@ ifeq ($(HAVE_X11), 1)
    ifeq ($(HAVE_XCB),1)
       LIBS += -lX11-xcb
    endif
+   ifeq ($(HAVE_XI2),1)
+      LIBS += -lXi
+      DEFINES += -DHAVE_XI2
+   endif
    ifeq ($(HAVE_XSCRNSAVER),1)
       LIBS += -lXss
    endif