fix build for --disable-clients --enable-simple-clients

fix build for --disable-clients --enable-simple-clients

Simple clients were relying on AM_CFLAGS and AM_CPPFLAGS set for
toytoolkit clients. With toytoolkit clients disabled, the build fails
with missing wayland-client.h.

Move AM_CFLAGS and AM_CPPFLAGS outside of conditional sections, since
they are meant to be global settings.

Let simple clients override AM_CPPFLAGS with their own
SIMPLE_CLIENT_CFLAGS, which the configure script already sets up for us,
but was unused until now.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>