Fix BSD .include etc syntax in Makefiles

Fix BSD .include etc syntax in Makefiles

* lisp/progmodes/make-mode.el (makefile-bsdmake-statements): Fix
the BSD conditional syntax (bug#24000).
(makefile-make-font-lock-keywords): Allow calling without keywords.
(makefile-bsdmake-font-lock-keywords): Add the conditional syntax.

     Makefile inclusion, conditional structures and for loops reminiscent of
     the C programming language are provided in make.  All such structures are
     identified by a line beginning with a single dot (`.') character.
     Whitespace characters may follow this dot, e.g.,

           .include <file>
     and
           .   include <file>

     are identical constructs