Fix out-of-tree build.

Fix out-of-tree build.

Use libtool "noinst" local helper libs and use one Makefile.am per
subdir, which is the usual/preferred method. These helper libraries are
purely local and will not be installed.

This also fixes out-of-tree builds of sigrok, i.e. building in a
directory other than the sigrok source directory, e.g.

 $ cd /home/user
 $ git clone ...sigrok
 $ cd sigrok
 $ ./autogen.sh
 $ mkdir /tmp/foo
 $ cd /tmp/foo
 $ /home/user/sigrok/configure
 $ make
 $ make install

This will place all build results (.o files, .la files, etc) in the
local build directory (/tmp/foo) instead of the source directory
(/home/user/sigrok in this example). The installation directory is
selected via the --prefix configure option (/usr/local per default).