flua: Always build as a bootstrap tool

We want to use flua from the source tree (not the host) during package
build, firstly to protect us from breaking changes in lua or libucl,
and secondly to allow (in future) cross-building of packages from
Linux or macOS.

Since we don't know if the user will be building packages during the
bootstrap phase, and because flua is fairly small and generally useful,
build it as a bootstrap tool unconditionally.

MFC after:	2 weeks
Reviewed by:	kevans, emaste
Sponsored by:	https://www.patreon.com/bsdivy
Differential Revision:	https://reviews.freebsd.org/D56270

Makefile.inc1

@@ -2614,17 +2614,11 @@ _bt_libkldelf_depend=${_bt}-lib/libkldelf
 _kldxref=	usr.sbin/kldxref
 ${_bt}-usr.sbin/kldxref: ${_bt_libelf_depend} ${_bt_libkldelf_depend}
 
-# flua is required to regenerate syscall files.  It first appeared during the
-# 13.0-CURRENT cycle, thus needs to be built on -older releases and stable
-# branches.
-.if ${BOOTSTRAPPING} < 1300059
+# flua is required to regenerate syscall files, and for the pkgbase build.
+# To protect us from changes in newer versions of flua or lua libraries
+# that might be installed on the host, always bootstrap it.
 ${_bt}-libexec/flua: ${_bt}-lib/liblua
-_flua= lib/liblua libexec/flua
-.if ${BOOTSTRAPPING} == 0
-_flua+= lib/libmd lib/libucl lib/libyaml
 ${_bt}-libexec/flua: ${_bt}-lib/libmd ${_bt}-lib/libucl ${_bt}-lib/libyaml
-.endif # BOOTSTRAPPING == 0
-.endif
 
 # r245440 mtree -N support added
 # r313404 requires sha384.h for libnetbsd, added to libmd in r292782
@@ -2911,7 +2905,11 @@ bootstrap-tools: ${_bt}-links .PHONY
     ${_elftoolchain_libs} \
     ${_libkldelf} \
     ${_kldxref} \
+    lib/liblua \
+    lib/libmd \
     lib/libopenbsd \
+    lib/libucl \
+    lib/libyaml \
     usr.bin/mandoc \
     usr.bin/rpcgen \
     ${_yacc} \
@@ -2921,7 +2919,7 @@ bootstrap-tools: ${_bt}-links .PHONY
     usr.bin/xinstall \
     ${_gensnmptree} \
     usr.sbin/config \
-    ${_flua} \
+    libexec/flua \
     ${_crunchide} \
     ${_crunchgen} \
     ${_etdump} \