Add support for running respack.lua with system Lua

Makefile.inc.in

@@ -135,4 +135,5 @@ BIN_MKDIR    = mkdir
 BIN_MKDIR_P  = mkdir -p
 BIN_ECHO     = echo
 BIN_TOUCH    = touch
+BIN_LUA      = @LUA@
 BIN_WX_CONFIG = @WX_CONFIG_PATH@

configure.ac

@@ -396,6 +396,7 @@ AS_IF([test x$with_system_luajit = xno], [],
        with_system_luajit="no"])])
 
 # Check if it's compiled in Lua 5.2 mode
+# This check can be skipped by manually setting agi_cv_luajit_is_52
 AS_IF([test x$with_system_luajit != xno], [
   aegisub_save_LIBS="$LIBS"
   aegisub_save_CPPFLAGS="$CPPFLAGS"
@@ -428,6 +429,14 @@ AS_IF([test $with_system_luajit = no],
       [AC_SUBST([LUAJIT_CFLAGS], ['-I$(TOP)vendor/luajit/include'])
        AC_SUBST([LUAJIT_LIBS], ['$(TOP)vendor/luajit/src/libluajit.a'])])
 
+# We also need a Lua binary to run part of the build system
+# Which version doesn't matter as the scripts are portable between them
+AC_CHECK_PROGS([LUA], [lua luajit lua5.3 lua-5.3 lua5.2 lua-5.2 lua5.1 lua-5.1])
+
+# If the user doesn't have an installed copy of Lua, just use the one built
+# as part of building LuaJIT
+AS_IF([test -z $LUA], [LUA="$srcdir/vendor/luajit/src/host/minilua"])
+
 ######################################################
 # Debugging support
 ######################################################

src/libresrc/Makefile

@@ -10,7 +10,7 @@ resrc_OBJ := \
 
 $(resrc_OBJ): $(d)default_config.h $(d)bitmap.h $(d)default_config.cpp $(d)bitmap.cpp
 
-RESPACK := cd $(TOP)src/libresrc; $(TOP)vendor/luajit/src/host/minilua $(TOP)tools/respack.lua
+RESPACK := cd $(TOP)src/libresrc; $(BIN_LUA) $(TOP)tools/respack.lua
 
 $(d)bitmap.cpp: $(d)bitmap.h
 $(d)default_config.cpp: $(d)default_config.h

tools/Makefile

@@ -12,6 +12,6 @@ repack-thes-dict_CPPFLAGS := -I$(TOP) -I$(TOP)libaegisub/include $(CFLAGS_ICU)
 
 PROGRAM += $(d)repack-thes-dict
 
-$(TOP)tools/respack.lua: $(TOP)vendor/luajit/src/host/minilua
+$(TOP)tools/respack.lua: $(shell command -v "$(BIN_LUA)")
 
 include $(TOP)Makefile.target

tools/respack.lua

@@ -1,4 +1,4 @@
-#!../vendor/luajit/src/host/minilua
+#!/usr/bin/env luajit
 -- Copyright (c) 2014, Thomas Goyne <plorkyeran@aegisub.org>
 --
 -- Permission to use, copy, modify, and distribute this software for any