configure.ac: mkdir $(objects)/ when building wayland protocols

The $(objects) directory (usually build/) may not have been created by
the time the wayland-scanner protocol files are being compiled. The
$(gen) directory is explicitly made with mkinstalldir, but the final
object file (and gcc dependency files) need to go into $(objects).

For whatever reason, this only ever seemed to occur if --disable-shared
was set.

Note that this commit doesn't regenerate ./configure, as there were a
few unexplained, unrelated differences my version of autoconf created,
as as an autotools novice, I didn't want to poke that bear just yet.

This hopefully should fix #3689

configure.ac

@@ -4383,6 +4383,7 @@ if test x$video_wayland = xyes; then
         printf '%s\n' "	\\$(RUN_CMD_GEN)\\$(WAYLAND_SCANNER) \\$(WAYLAND_SCANNER_CODE_MODE) \\$< \\$@" ;\
         echo ;\
         printf '%s\n' "\\$(objects)/\$p-protocol.lo: \\$(gen)/\$p-protocol.c \\$(gen)/\$p-client-protocol.h" ;\
+        printf '%s\n' "	@\\$(SHELL) \\$(auxdir)/mkinstalldirs \\$(objects)" ;\
         printf '%s\n' "	\\$(RUN_CMD_CC)\\$(LIBTOOL) --tag=CC --mode=compile \\$(CC) \\$(CFLAGS) \\$(EXTRA_CFLAGS) $DEPENDENCY_TRACKING_OPTIONS -c \\$< -o \\$@" ;\
         done ;\
         echo ;\