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.
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