Add guard for out-of-tree builds

[Graham Bloice <graham.bloice () trihedral com>]

With the issue of out-of-tree builds picking up the -in-tree copy of
config.h due to the use of #include "config.h", I wondered if we should put
a guard in config.h such that if it is used by an out-of-tree build an
immediate warning is issued, e.g.

#ifndef IN_TREE_BUILD
#error Using __FILE__ in an out-of-tree build
#endif

And ensuring that in-tree builds define IN_TREE_BUILD.

Is there any value in this, are there other files that should also be
guarded and is this possible with autconf\automake?


-- 
Graham Bloice

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