package/x11r7/xlib_libxshmfence: disable on riscv32

xlib_libxshmfence unconditionally uses SYS_futex which raises the
following build failure on riscv32:

xshmfence_futex.h:58:17: error: 'SYS_futex' undeclared (first use in this function); did you mean 'sys_futex'?
   58 |  return syscall(SYS_futex, addr1, op, val1, timeout, addr2, val3);
      |                 ^~~~~~~~~
      |                 sys_futex

Fixes:
 - http://autobuild.buildroot.org/results/b3523e35fde0fac04b96a6278cbc6ffdfe56f7d1

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/mesa3d/Config.in

@@ -65,7 +65,8 @@ config BR2_PACKAGE_MESA3D_DRI_DRIVER
 		!BR2_PACKAGE_MESA3D_OPENGL_GLX && \
 		!BR2_PACKAGE_MESA3D_OPENGL_EGL
 	select BR2_PACKAGE_XLIB_LIBXSHMFENCE if \
-		(BR2_PACKAGE_XORG7 && BR2_TOOLCHAIN_HAS_SYNC_4)
+		(BR2_PACKAGE_XORG7 && BR2_TOOLCHAIN_HAS_SYNC_4 && \
+			!BR2_RISCV_32)
 
 config BR2_PACKAGE_MESA3D_VULKAN_DRIVER
 	bool

package/x11r7/xlib_libxshmfence/Config.in

@@ -1,5 +1,8 @@
 config BR2_PACKAGE_XLIB_LIBXSHMFENCE
 	bool "libxshmfence"
+	# Due to use of SYS_futex - can be enabled again when upstream
+	# adds SYS_futex64 as an alternative
+	depends on !BR2_RISCV_32
 	depends on BR2_TOOLCHAIN_HAS_SYNC_4
 	select BR2_PACKAGE_XORGPROTO
 	help

package/x11r7/xserver_xorg-server/Config.in

@@ -53,7 +53,8 @@ config BR2_PACKAGE_XSERVER_XORG_SERVER_MODULAR
 	depends on BR2_INSTALL_LIBSTDCPP
 	select BR2_PACKAGE_LIBDRM
 	select BR2_PACKAGE_LIBPCIACCESS
-	select BR2_PACKAGE_XLIB_LIBXSHMFENCE if BR2_TOOLCHAIN_HAS_SYNC_4
+	select BR2_PACKAGE_XLIB_LIBXSHMFENCE if \
+		(BR2_TOOLCHAIN_HAS_SYNC_4 && !BR2_RISCV_32)
 	help
 	  This variant of the X.org server is the full-blown variant,
 	  as used by desktop GNU/Linux distributions. The drivers (for