lto-plugin: make claim_file_handler thread-safe

lto-plugin/ChangeLog:

	* lto-plugin.c (plugin_lock): New lock.
	(claim_file_handler): Use mutex for critical section.
	(onload): Initialize mutex.
	* configure.ac: Check for pthread.h.
	* configure: Regenerate.
	* config.h.in: Regenerate.