Use a standard autotools setup.

fx2lafw now uses a standard autoconf/automake based build and
configuration system like most other sigrok subprojects (instead of two
sets of custom, not very portable Makefiles).

 $ ./configure && make && make install

It now supports all the usual things one expects, for example:

 - The --prefix et. al. options to specify installation paths.

 - Standard set of 'make' targets, e.g. 'install', 'uninstall',
   'dist', etc.

 - In-tree builds as well as out-of-tree builds.

 - Works pretty much everywhere portably, regardless of 'make'
   implementation (GNU Make or BSD make or...) and regardless of shell
   (bash, sh, dash, ksh, csh, ...).

 - The build output matches the usual 'silent rules' output of other
   (sigrok / autotools) projects. Using 'make V=1' one can get the full
   build output.