Drop use_lib64

Drop use_lib64 from jhbuildrc and stop using --libdir.

We now never pass --libdir= to build systems, which means that libraries
will always be installed into the default location of ${prefix}/lib.

We replace use_lib64 with a new variable called 'system_libdirs' which
is a list of system library paths.  The default value is determined in a
system-specific way which knows about /usr/local/lib, /usr/lib64 and
Debian-style multiarch directories.

We keep a few odd bits around for backwards compatibility reasons such
as the ${libdir} expansion and JHBUILD_LIBDIR environment variable.

Some examples of the detected default value on various systems:

  Ubuntu:     ['/usr/lib/x86_64-linux-gnu', '/usr/lib']
  Fedora:     ['/usr/lib64']
  FreeBSD:    ['/usr/local/lib', '/usr/lib']