meson: Fix warnings when reading back and checking feature flags

Meson does not like comparing things of different types which is a
problem when reading back values of feature flags as they may contain
either false (bool) or 1 (string).

Since there is a strong reason why we use false when the feature does
not exist, we work around this issue by always converting the returned
value to int via to_int().

Fixes: xorg/xserver#1190
Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
5 jobs for meson-feature-flag-checks in 5 minutes and 24 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Docker Image
passed debian-buster #11516027

00:00:22

 
  Build And Test
passed autotools #11516028

00:05:01

passed meson #11516030

00:02:40

passed meson-noglamor #11516032

00:01:53

passed mingw-cross-build #11516034

00:01:06