Properly detect working jobserver in gcc driver.

Properly detect working jobserver in gcc driver.

2019-08-02  Martin Liska  <mliska@suse.cz>

	PR lto/91313
	* gcc.c (driver::maybe_run_linker): Call detect_jobserver
	to detect working job server.
	(driver::detect_jobserver): Test whether jobserver
	is active from GCC driver. That will prevent situation where
	GCC is invoked from a LD plugin and the linker already uses
	file descriptors suggested by make.  That leads to a wrong
	detection.
	* gcc.h (driver): Add detect_jobserver.
	* lto-wrapper.c (jobserver_active_p): Simplify sscanf by
	not scanning for --jobserver-auth prefix.

From-SVN: r274003