libXt: util: don't link makestrs with target cflags

The line:

AM_CFLAGS = $(XT_CFLAGS)

in util/Makefile.am is wrong because it adds target cflags to the
compilation of makestrs, which is built for the build machine, which
leads to build failures when cross-compiling.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Retrieved from:
https://git.buildroot.net/buildroot/tree/package/x11r7/xlib_libXt/0001-util-don-t-link-makestrs-with-target-cflags.patch

]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>