gyp file: Remove the ".lib" suffix from "-luser32.lib" and document.
gyp file: Remove the ".lib" suffix from "-luser32.lib" and document.

gyp tolerates all of:

  'user32'
  'user32.lib'
  '-luser32'
  '-luser32.lib'

With the --format=make generator, the exact string value is passed to gcc,
which only accepts the '-luser32' syntax.  The MSVC generator is also
happy with this format, so that's what I'll use.

With this change, it's possible to use the gyp file to build the C++
components.  I'm not sure if that's useful, but why not?  The resulting
Makefile can be configured by passing settings like CXX and LDFLAGS on the
make command-line.

Also: document use of gyp a bit.

rprichard/winpty,rprichard/winpty,fracting/winpty,rprichard/winpty,fracting/winpty,rprichard/winpty,fracting/winpty,fracting/winpty