lto: pass -pthread to AM_LDFLAGS [PR 106118]

Move -pthread from configure.ac to Makefile.in so that it is passed to AM_LDFLAGS.

	PR lto/106118

lto-plugin/ChangeLog:

	* configure.ac: Move -pthread from here...
	* Makefile.am: ...to here.
	* configure: Regenerate.
	* Makefile.in: Likewise.