Relates to ArcticaProject/nx-libs#890.

This file has been deleted, but references to it in the build remain,
causing `debuild` to fail.

nx-libs.spec: Dropping nxdialog files from installation for now when building RPMs. Postponing nxdialog integration with RPM packaging for now.

nx-libs.spec: newer versions of SLE* and OpenSuSE use xkbcomp-devel, while RedHat-based distros want xorg-x11-xkb-utils-devel for xkbcomp.pc.

What actually happened was that MFLAGS were passed-through more
correctly, which forced make into jobserver mode (due to the inclusion
of --jobserver-fds=... in %{_smp_mflags}), but the processes were not
able to read from their FD's, so it defaulted to non-parallel builds
instead.

This utilizes IMAKE_DEFINES to pass down the value to UseTIRPC directly.

Fixes: ArcticaProject/nx-libs#665

Revert "nx-libs.spec: Starting with Fedora 28 / RHEL 8, SunRPC support is being split out into a separate libraries (libtirpc)."

This reverts commit 9f93e96e.

nx-libs.spec: Starting with Fedora 28 / RHEL 8, SunRPC support is being split out into a separate libraries (libtirpc).

Fixes ArcticaProject/nx-libs#665.

These headers have been obsoleted in glibc and are not shipped by
default any longer. Fedora follows suit.

Fixes RPM builds building libNX_X11 with poll() support, even though we
explicitly disable it using --disable-poll in the main Makefile.

Fixes: ArcticaProject/nx-libs#540

I didn't mean to drop it in the first place in
07d30835 and
71bda0ed.

Dropping the sed hack was fine -  but we actually have to add these
flags to CDEBUGFLAGS as well.

This reverts commit cc47066b.

This package is actually already depended upon.

shmproto.h must be in some other package. Investigating.

nx-libs.spec: imake is part of xorg-x11-util-devel on older *SUSE versions and the splitted imake package on later versions.

Partly reverts ec30a857, but in a
forward-compatible manner.

Legacy libXfont1 versions are automatically detected by the main
Makefile.