Port xcrun configuration to GNU/Linux.
* 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.
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