Support Lua 5.3

No code changes, only need to add appropriate compatibility defines and
change a version number check.

Also update to latest ax_lua.m4, which includes useful fixes as well as
5.3 support.

README

@@ -40,7 +40,7 @@ system that supports more than one version of Lua):
 
 For example, on Debian or Ubuntu:
 
-  CPPFLAGS='-I/usr/include/lua5.1' ./configure --libdir=/usr/local/lib/lua/5.1 --datadir=/usr/local/share/lua/5.1 --with-lua-suffix=5.1
+  LUA=lua5.1 CPPFLAGS='-I/usr/include/lua5.1' ./configure --libdir=/usr/local/lib/lua/5.1 --datadir=/usr/local/share/lua/5.1
 
 To run some tests:
 

configure.ac

@@ -36,12 +36,9 @@ esac
 AC_CHECK_LIB([ffi], [ffi_closure_alloc], , AC_MSG_FAILURE([cannot find new enough libffi]))
 AC_CHECK_HEADERS([stdint.h])
 
-dnl Lua 5.1
-AX_PROG_LUA(501, 503)
+dnl Lua >= 5.1
+AX_PROG_LUA(5.1, 5.4)
 AX_LUA_HEADERS
-AX_LUA_HEADERS_VERSION(501, 503)
-AC_SUBST(LUA)
-AC_SUBST(LUA_INCLUDE)
 
 dnl md5sum
 AX_WITH_PROG([MD5SUM], [md5sum], [false])