Move the -I options for all our source subdirectories out of AM_CFLAGS

Move the -I options for all our source subdirectories out of AM_CFLAGS
and into AM_CPPFLAGS. This is more conceptually sensible according to
my reading of the automake manual, and also has the specific desirable
effect that they move to the front of the command line, ahead of any
'system' type -I options that autoconf might have felt a need for.

A user reported that autoconf had added -I/usr/local/include to their
command line for the sake of a required header file, but their
/usr/local/include also turned out to include a thing called 'proxy.h'
(from libproxy, nothing to do with us) which shadowed our own proxy.h
and caused a build failure. This should fix that.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9736 cda61777-01e9-0310-a592-d414129be87e