lto-plugin: use -pthread only for detected targets

Use -pthread only if we are going to use pthread functionality.

	PR bootstrap/106156

lto-plugin/ChangeLog:

	* Makefile.am: Use ac_lto_plugin_extra_ldflags for AM_LDFLAGS.
	* configure.ac: Use AC_SUBST(ac_lto_plugin_extra_ldflags).
	* Makefile.in: Regenerate.
	* configure: Regenerate.