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* configure.ac (xcsdkdir): Default to empty.
(XCRUN): Don't require Darwin for xcrun.  Move xcrun checking to
just before AM_INIT_AUTOMAKE, to make the dependency between it
and automake clearer.
(CC): Don't use AC_PROG_CC twice; only the first use expands to the
shell code that is wanted, which breaks 'configure' on non-Darwin
platforms.  Instead, fix CC by hand if it's not found.