LJ_GC64: Add build options and install instructions.

doc/install.html

@@ -175,6 +175,14 @@ MSVC or WinSDK.</li>
 Please read the instructions given in these files, before changing
 any settings.
 </p>
+<p>
+LuaJIT on x64 currently uses 32 bit GC objects by default.
+<tt>LJ_GC64</tt> mode may be explicitly enabled:
+add <tt>XCFLAGS=-DLUAJIT_ENABLE_GC64</tt> to the make command or run
+<tt>msvcbuild gc64</tt> for MSVC/WinSDK. Please check the note
+about the <a href="extensions.html#string_dump">bytecode format</a>
+differences, too.
+</p>
 
 <h2 id="posix">POSIX Systems (Linux, OSX, *BSD etc.)</h2>
 <h3>Prerequisites</h3>
@@ -584,14 +592,11 @@ intend to load Lua/C modules at runtime.
 </li>
 <li>
 If you're building a 64 bit application on OSX which links directly or
-indirectly against LuaJIT, you need to link your main executable
-with these flags:
+indirectly against LuaJIT which is not built for <tt>LJ_GC64</tt> mode,
+you need to link your main executable with these flags:
 <pre class="code">
 -pagezero_size 10000 -image_base 100000000
 </pre>
-Also, it's recommended to <tt>rebase</tt> all (self-compiled) shared libraries
-which are loaded at runtime on OSX/x64 (e.g. C extension modules for Lua).
-See: <tt>man rebase</tt>
 </li>
 </ul>
 <p>Additional hints for initializing LuaJIT using the C API functions:</p>