Disable libitm if unsupported

[Richard Henderson]

As suggested by Joseph, this consolidates the knowledge of libitm
support in the libitm directory.  And better, in a file not controlled
by autoconf, so less fumbling around for the correct edition.  ;-)

Committed in two hunks because I faffed it the first time and managed
to not notice that I'd disabled libitm on supported systems too.

Tested on x86_64-linux and ppc64-linux.


r~
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