package/x11r7/xlib_libxshmfence: set shared-memory-dir

libxshmfence has multiple paths available for the shared memory
directory, as some distros [0] opt to mount their tmpfs in a
non-standard location such as /run/shm rather than /dev/shm.

The default value of 'auto' will set this path to whatever the host is
using, leaking host configuration into the target. See [1].

With X configurations that depend on shared memory files for futexes,
(muvm [2] is a notable example), this results in applications silently
breaking during presentation with a blank window, as the configured
path doesn't have the required tmpfs mount.

Set this path explicitly to avoid situations where the host context
leaks into the package build, causing feature breakage.

[0] https://wiki.ubuntu.com/OneiricOcelot/ReleaseNotes?action=show&redirect=OneiricOcelot%2FTechnicalOverview#Upgrades
[1] https://gitlab.freedesktop.org/xorg/lib/libxshmfence/-/blob/libxshmfence-1.3.3/configure.ac#L144
[2] https://github.com/AsahiLinux/muvm

Signed-off-by: Joseph Kogut <joseph.kogut@gmail.com>
[Julien:
 - add link to shared memory dir detection code in commit log
 - replace "+=" by "=" in _CONF_OPTS
]
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/x11r7/xlib_libxshmfence/xlib_libxshmfence.mk

@@ -12,4 +12,7 @@ XLIB_LIBXSHMFENCE_LICENSE_FILES = COPYING
 XLIB_LIBXSHMFENCE_INSTALL_STAGING = YES
 XLIB_LIBXSHMFENCE_DEPENDENCIES = host-pkgconf xorgproto
 
+# default 'auto' leaks context from host
+XLIB_LIBXSHMFENCE_CONF_OPTS = --with-shared-memory-dir=/dev/shm
+
 $(eval $(autotools-package))