lua: add fallback to detect LuaJIT

lua: add fallback to detect LuaJIT

Before:
  LIBLUA_PKG_CONFIG_NAME=luajit ./configure --enable-lua

After:
  ./configure --enable-lua

In the previous versions, for enabling LuaJIT,
it needs to specify LIBLUA_PKG_CONFIG_NAME=luajit explicitly.

In this commit, add fallback to detect LuaJIT without specifying
LIBLUA_PKG_CONFIG_NAME=luajit explicitly by default (just use
--enable-lua).

Note that if both of lua and luajit exists, then lua is used.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>