Trying to build with meson with -Dzlib=disabled failed with the
following error message:

cairo/test/meson.build:599:2: ERROR: Unknown variable
"libcairoscript_dep".

This commit fixes that problem by adding a not-found dependency if
cairo-script is not built.

Additionally, follow-up problems are fixed:

- any2ppm.c still tried to include cairo-script-interpreter.h, which was
  not found
- Building cairo-test-trace was attempted, but that also failed because
  of missing script support.

Fixes: cairo/cairo#475


Signed-off-by: Uli Schlachter <psychon@znc.in>