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.

src/sysprof-cli/meson.build

@@ -6,11 +6,13 @@ sysprof_cli_c_args = [
 ]
 
 sysprof_cli_deps = [
-  dependency('polkit-agent-1'),
-
   libsysprof_static_dep,
 ]
 
+if polkit_agent_dep.found()
+  sysprof_cli_deps += polkit_agent_dep
+endif
+
 sysprof_cli = executable('sysprof-cli', sysprof_cli_sources,
   dependencies: sysprof_cli_deps,
         c_args: release_flags + sysprof_cli_c_args,