meson: Use feature options

By using meson features we can replace the handcrafted dependency
auto-detection by just passing the value from get_option directly to the
required arg for dependency, find_library etc.
'auto' features make the dependency optional, 'enabled' requires it
while 'disabled' features will skip detection entirely.

Any skipped or not found dependency will just be a no-op when passed to
build steps and therefore we can also skip the creation of empty vars.

The use of skip_deps for these is dropped here as meson provides a way
to disable all optional features in one go by passing
'-Dauto_features=disabled'.

.github/workflows/build_test.sh

@@ -11,7 +11,7 @@ ARGS=(
     "--optimization=0"
     "--optimization=s"
     "--optimization=3 -Db_lto=true -Ddns-over-tls=false"
-    "--optimization=3 -Db_lto=false -Dtpm2=false -Dlibfido2=false -Dp11kit=false"
+    "--optimization=3 -Db_lto=false -Dtpm2=disabled -Dlibfido2=false -Dp11kit=disabled"
     "--optimization=3 -Ddns-over-tls=openssl"
     "--optimization=3 -Dfexecve=true -Dstandalone-binaries=true -Dstatic-libsystemd=true -Dstatic-libudev=true"
     "-Db_ndebug=true"