The struct entry 'extension' is not really a (filename) extension, but
rather a unique ID used for input or output formats, e.g. in the sigrok
CLI or GUI interface. Thus, rename it accordingly.
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).