In the lib, we should only #include "sigrok.h" or "sigrok-internal.h",
but not the (possibly installed and thus different/older versions) via
<sigrok.h> or <sigrok-internal.h>.
Frontends should of course use <sigrok.h> and <sigrok-internal.h>.
We should use these (internal) functions in libsigrok exclusively from
now on, i.e. no more use of glib's g_debug() etc.
These functions are only for libsigrok, the frontends use whatever
logging mechanism is suitable there.
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).