pppd/Makefile.linux: Fix reproducibility issue with differing make versions (#234)

pppd/Makefile.linux: Fix reproducibility issue with differing make versions (#234)

We were seeing reproducibility issues where one host would use the internal
logwtmp wrapper, another would use the one in libutil. The issue was that in
some cases the "\#include" was making it to CC, in others, "#include". The
issue seems to be related to shell escaping.

The root cause looks to be:
http://git.savannah.gnu.org/cgit/make.git/commit/?id=c6966b323811c37acedff05b576b907b06aea5f4

Instead of relying on shell quoting, use make to indirect the variable
and avoid the problem.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>