Fix: standardize parser/lexer building

This patch makes the build system act as follows:

    if in Git repo:
        require bison/flex (configure)
        build parser/lexer (make, make clean)
    else:
        if bison exists:
            build parser (make, make clean)
        else:
            warn that bison is missing (configure)
            create "error" parser target in Makefile (make)
            do not clean parser (make clean not available)

        if flex exists:
            build lexer (make, make clean)
        else:
            warn that flex is missing (configure)
            create "error" lexer target in Makefile (make)
            do not clean lexer (make clean not available)

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>