[OE-core] race between aclocal and unstaging of .m4 files

[Phil Blundell]

If do_configure() from one recipe runs in parallel with
sysroot_cleansstate() from another then aclocal may fail because it
doesn't react very well to .m4 files disappearing underneath it.  This
manifests as slightly obscure failures such as:

| aclocal: error: aclocal: file '.../tmp-eglibc/sysroots/x86_64-linux/share/aclocal/alsa.m4' does not exist

where the .m4 file in question is not one that the recipe being built
would actually want to use.  (The alsa.m4 error above actually occurred
during a build of attr.)

There seem to be two obvious ways of fixing this:

a) Add more locking so that these things can't happen in parallel

b) Patch aclocal to make this situation non-fatal

My inclination would be to do (b) since it's less intrusive.  Any
objections or better suggestions?

p.