gtk configure fails to detect XInput2.h

Hi. I am currently running the configure script of gtk. It stops with the message: "configure: error: *** XInput2 extension not found. Check 'config.log' for more details.". Then I checked config.log. And there it is says: "configure:23790: error: *** XInput2 extension not found. Check 'config.log' for more details."  No very useful is it?

Then I did a little of online research and figured out that this extension belongs to the libXi package. I built it and installed it to a dir.

Finally, I executed the configure script again, passing the include dir to configure through the CPPFLAGS. 

CPPFLAGS="-I/media/34GB/Arquivos-de-Programas-Linux/xorg/X11-1.3.6/include/ -I/media/34GB/Arquivos-de-Programas-Linux/xorg/Xorgproto-2018.1/include/ -I/media/34GB/Arquivos-de-Programas-Linux/xorg/Xi-1.5.0/include/"

And the result was: "configure: error: *** XInput2 extension not found. Check 'config.log' for more details." I checked the include dir of Xi. Inside it there is a X11 dir, inside of it an extensions dir, and inside it, XInput.h and XInput2.h. So, it seems ok to me

I then gave alook at configure.ac. This is what: "grep XInput* configure.ac" returned:

# set up things for XInput
    AC_CHECK_HEADER(X11/extensions/XInput2.h,
                    AC_DEFINE(XINPUT_2, 1, [Define to 1 if XInput 2.0 is available]))
                       AC_DEFINE(XINPUT_2_2, 1, [Define to 1 if XInput 2.2 is available]),
                       [[#include <X11/extensions/XInput2.h>]])])
        [AC_MSG_ERROR([*** XInput2 extension not found. Check 'config.log' for more details.])])

It might be a version issue? I gave a quick glance inside XInput2.h and did not see any definition inside it.
From libXi.la there is:

# Version information for libXi.
current=7
age=1
revision=0

and xi.pc says:
Version: 1.5.0

So, any ideas? Thanks for your time.