lto: Fix empty fnctl.h build error with MinGW.

MSYS2+MinGW contains headers without defining expected contents.
This fix checks that the fcntl function is actually defined.

Bootstrapped/regtested on x86_64-linux. Committed as obvious.

gcc/ChangeLog:

	* lockfile.cc (LOCKFILE_USE_FCNTL): New.
	(lockfile::lock_write): Use LOCKFILE_USE_FCNTL.
	(lockfile::try_lock_write): Use LOCKFILE_USE_FCNTL.
	(lockfile::lock_read): Use LOCKFILE_USE_FCNTL.
	(lockfile::unlock): Use LOCKFILE_USE_FCNTL.
	(lockfile::lockfile_supported): Use LOCKFILE_USE_FCNTL.