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/sysprof-cli.c

@@ -32,9 +32,11 @@
 
 #include <sysprof.h>
 
-#define POLKIT_AGENT_I_KNOW_API_IS_SUBJECT_TO_CHANGE
-#include <polkit/polkit.h>
-#include <polkitagent/polkitagent.h>
+#if HAVE_POLKIT_AGENT
+# define POLKIT_AGENT_I_KNOW_API_IS_SUBJECT_TO_CHANGE
+# include <polkit/polkit.h>
+# include <polkitagent/polkitagent.h>
+#endif
 
 #include "sysprof-capture-util-private.h"
 
@@ -266,8 +268,10 @@ int
 main (int   argc,
       char *argv[])
 {
+#if HAVE_POLKIT_AGENT
   PolkitAgentListener *polkit = NULL;
   PolkitSubject *subject = NULL;
+#endif
   g_autoptr(SysprofCaptureWriter) writer = NULL;
   g_autoptr(SysprofProfiler) profiler = NULL;
   g_autofree char *power_profile = NULL;
@@ -413,6 +417,7 @@ Examples:\n\
 
   main_loop = g_main_loop_new (NULL, FALSE);
 
+#if HAVE_POLKIT_AGENT
   /* Start polkit agent so that we can elevate privileges from a TTY */
   if (g_getenv ("DESKTOP_SESSION") == NULL &&
       (subject = polkit_unix_process_new_for_owner (getpid (), 0, -1)))
@@ -434,6 +439,7 @@ Examples:\n\
                       pkerror->message);
         }
     }
+#endif
 
   /* Warn about access if we're in a container */
   if (g_file_test ("/.flatpak-info", G_FILE_TEST_EXISTS))