Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

tools/build/bootstrap-m4: regenerate after d37f81e35b

Reviewed by: jkim

Revert "contrib/flex: Drop local __dead2 patch"

I missed the review comment to commit the changes separately, will
reland in multiple smaller commits.

Requested By: jkim
This reverts commit bbd421cdf6d8c6102e6fd3979c5bec21ace3c2e3.

contrib/flex: Drop local __dead2 patch

Upstream flex has added a yynoreturn, so this diff is no longer needed.
Partially reverts r181269. Also regenerate the pre-generated files that
are used for bootstrapping.

Reviewed By: jkim

Differential Revision: https://reviews.freebsd.org/D29679

Add necessary Makefile.inc1 infrastructure for building on non-FreeBSD

The most awkward bit in this patch is the bootstrapping of m4:
We can't simply use the host version of m4 since that is not compatible
with the flags passed by lex (at least on macOS, possibly also on Linux).
Therefore we need to bootstrap m4, but lex needs m4 to build and m4 also
depends on lex (which needs m4 to generate any files). To work around this
cyclic dependency we can build a bootstrap version of m4 (with pre-generated
files) then use that to build the real m4.

This patch also changes the xz/unxz/dd tools to always use the host version
since the version in the source tree cannot easily be bootstrapped on macOS
or Linux.

Reviewed By: brooks, imp (earlier version)
Differential Revision: https://reviews.freebsd.org/D25992