Turn polkit-agent support into an optional feature.

This simplifies deployment on embedded devices, where polkit is usually
unncessary at runtime, but pulls in quite a few otherwise unncessary
dependencies. Start to improve the situation by allowing to selectively
disable polkit-agent support at compile time, which aids in container
usage scenarios, where one wants to invoke 'sysprof-cli' from within
the container. Bypassing polkit-agent in the container is then desired,
since the host sysprofd will handle asking for permissions to enable
the tracing. It allows for a simpler setup of rootless podman
containers, avoiding UID mismatches, that lead to rejection of the
tracing enablement.

- Add a new 'polkit-agent' meson build feature, that allows to force disabling
  polkit-agent support (-Dpolkit-agent=disabled).

- Mark the 'polkit-agent' feature as enabled, by default, to reflect
  the current status (sysprof-cli did not build without polkit-agent support).

- libsysprof/sysprof-instrument.c: Build fix when polkit is not available,
  remove the unnecessary 'g_autopr(PolkitDetails) details' variable.

- Alter the sysprof-cli dependencies to only attempt to link against
  polkit-agent, if necessary. Modify sysprof-cli.c to wrap all code using
  polkit-agent in HAVE_POLKIT_AGENT blocks.

meson_options.txt

@@ -11,6 +11,12 @@ option('gtk', type: 'boolean')
 # Allow disabling the installation of libsysprof-capture*.a
 option('install-static', type: 'boolean')
 
+# Allow disabling of features that depend on polkit-agent.
+option('polkit-agent', type: 'feature',
+  value: 'auto',
+  description: 'Enable features which require polkit-agent-1'
+)
+
 # Optionally compile sysprofd, which is needed to get elevated privileges.
 # You only really want to ignore this if you are running from a container
 # and are talking to a host daemon. Also, if you're compiling for something