Since on Intel CET enabled host, dlopen in Intel CET enabled applications
fails on shared libraries which aren't Intel CET enabled, compile with
-fcf-protection on Intel CET enabled host when jit is enabled to enable
Intel CET on libgccjit.

	* Makefile.in (CET_HOST_FLAGS): New.
	(COMPILER): Add $(CET_HOST_FLAGS).
	* configure.ac: Add GCC_CET_HOST_FLAGS(CET_HOST_FLAGS) and
	AC_SUBST(CET_HOST_FLAGS).  Clear CET_HOST_FLAGS if jit isn't
	enabled.
	* aclocal.m4: Regenerated.
	* configure: Likewise.