RFC: Separate build dir for autotools

[Richard Purdie <richard.purdie@linuxfoundation.org>]

For a while poky and others have been using the seperatebuilddir.inc
file. This makes ${S} != ${B} and means that ${B} can be wiped when
configuration changes.

This helps deal with the fact that autotools and friends are not
particular good at rebuilding things that change. The recent libcheck
upgrade which broke bluez4 builds is but one example of the kind of
problem we can hit. Sadly bluez4 doesn't work for S!=B so it didn't
protect against that issue but it does protect other packages.

I'd really love to switch the default in autotools.bbclass to make this
the default and then have broken packages opt out of it.

I can deal with OE-Core and make sure the metadata there is updated, my
bigger worry is what this would do to other layers.

The options I can see are therefore basically:

a) We change the default in autotools.bbclass and fix things that break

b) We introduce an "autotools2" class and have recipes inherit that.
This version would default to separate builddirs. It does mean change to
all the "good" recipes rather than the broken ones though and hence a
fair bit of churn.

Have we got the willpower to fix a)? is b) an option? Other ideas?

One way or another I think we should try and switch things somehow...

Cheers,

Richard