cross-compiles need not build host native lua library

Because some Makefile.top dependencies triggered the build
of the host native lua library, the cross-compiles were
building it needlessly.

Make it a make variable so that it can be overridden by
cross-compile recipes in sys/unix/hints/include/cross-*.2020

sys/unix/Makefile.top

@@ -94,6 +94,7 @@ DATNODLB = $(VARDATND) license symbols
 DATDLB = $(DATHELP) dungeon.lua tribute $(SPEC_LEVS) $(QUEST_LEVS) $(VARDATD)
 DAT = $(DATNODLB) $(DATDLB)
 
+TOPLUALIB = lib/lua/liblua.a
 ALLDEP = $(GAME) recover Guidebook $(VARDAT) spec_levs check-dlb
 
 # first target is also the default target for 'make' without any arguments
@@ -104,7 +105,7 @@ all:    $(ALLDEP)
 $(GAME): lua_support
 	( cd src ; $(MAKE) $(GAME) )
 
-lua_support: lib/lua/liblua.a include/nhlua.h
+lua_support: $(TOPLUALIB) include/nhlua.h
 	@true
 lib/lua-$(LUA_VERSION)/src/liblua.a: lib/lua-$(LUA_VERSION)/src/lua.h
 	( cd lib/lua-$(LUA_VERSION)/src \
@@ -112,7 +113,7 @@ lib/lua-$(LUA_VERSION)/src/liblua.a: lib/lua-$(LUA_VERSION)/src/lua.h
 lib/lua/liblua.a: lib/lua-$(LUA_VERSION)/src/liblua.a
 	@( if [ ! -d lib/lua ] ; then mkdir -p lib/lua ; fi )
 	cp lib/lua-$(LUA_VERSION)/src/liblua.a $@
-include/nhlua.h: lib/lua/liblua.a
+include/nhlua.h: $(TOPLUALIB)
 	echo '/* nhlua.h - generated by top Makefile */' > $@
 	@echo '#include "../lib/lua-$(LUA_VERSION)/src/lua.h"' >> $@
 	@sed -e '/(lua_error)/!d' -e '/(lua_error)/s/;/ NORETURN;/1' \
@@ -207,7 +208,7 @@ dlb:
 	( cd util ; $(MAKE) dlb )
 	( cd dat ; LC_ALL=C ; ../util/dlb cf nhdat $(DATDLB) )
 
-wasm:
+wasm: include/nhlua.h
 	( cd src ; $(MAKE) $(WASM_TARGET) )
 
 package: $(GAME) recover $(VARDAT) spec_levs

sys/unix/hints/include/cross-pre.2020

@@ -184,6 +184,7 @@ override SYSOBJ= $(TARGETPFX)pcmain.o $(TARGETPFX)msdos.o \
 		$(TARGETPFX)tileset.o $(TARGETPFX)tile.o
 override WINLIB=
 override LUALIB=
+override TOPLUALIB=
 override GAMEBIN = $(TARGETPFX)nethack.exe
 override PACKAGE = dospkg
 override PREGAME += mkdir -p $(TARGETDIR) ;
@@ -273,6 +274,7 @@ override SYSOBJ = $(TARGETPFX)amidos.o $(TARGETPFX)amigst.o \
 #		../util/txt2iff.o
 override WINLIB=
 override LUALIB=
+override TOPLUALIB=
 override GAMEBIN = $(TARGETPFX)nethack
 override PACKAGE = amigapkg
 override PREGAME += mkdir -p $(TARGETDIR) ;
@@ -375,6 +377,7 @@ override SYSOBJ= $(TARGETPFX)libnethackmain.o \
 		$(TARGETPFX)winshim.o
 override WINLIB = emranlib 
 override LUALIB=
+override TOPLUALIB=
 override REGEXOBJ = $(TARGETPFX)posixregex.o
 override WINOBJ=
 # don't bother Making regular NetHack executable