call libtoolize as glibtoolize; use dylib shared library suffix

This gets amrnb.js building on Mac. There are still some warnings, a bunch of which look like this:

> amrnb/Makefile.am:58: warning: source file '$(DEC_SRC_DIR)/amrdecode.cpp' is in a subdirectory,
> amrnb/Makefile.am:58: but option 'subdir-objects' is disabled

And then there are these three:

> WARNING  root: emcc: cannot find library "m"
> WARNING  root: emcc: cannot find library "c"
> WARNING  root: Dynamic libraries (.so, .dylib, .dll) are currently not supported by Emscripten. For build system emulation purposes, Emscripten will now generate a static library file (.bc) with the suffix '.dylib'. For best practices, please adapt your build system to directly generate a static LLVM bitcode library by setting the output suffix to '.bc.')

So there may be more work to do here. Nevertheless, the build succeeds.

The two changes are:

1. Call *libtoolize* as *glibtoolize* on Mac, per [this StackOverflow question](http://stackoverflow.com/questions/15448582/installed-libtool-but-libtoolize-not-found), which says, "You typically need to use glibtool and glibtoolize, since libtool already exists on OS X as a binary tool for creating Mach-O dynamic libraries."
2. Change the shared library suffix from *so* to *dylib* on Mac.