Add setcap helper

This will setcap the perf stats helper appropriately when installed by
root. It wont fail if you try an unprivileged install, but it means the
perf stats helper also wont work.

Also check that we have the CAP_PERFMON capability in the kernel as this
isnt supported on ubuntu 20. On this platform packagers should setuid as
root.

src/app/setcap-helper.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+# Simple post-install script to setcap the metrics helper
+
+setcap="${1?SETCAP_UNSET}"
+bindir="${2?BINDIR_UNSET}"
+exe="${3?EXE_UNSET}"
+exe_fullpath="$MESON_INSTALL_DESTDIR_PREFIX/$bindir/$exe"
+
+euid=$(id -u)
+if [ $euid != "0" ]; then
+	echo "Warning: installed by unprivileged user, could not setcap $exe_fullpath" >&2
+	exit 0
+fi
+if ! capsh --supports=CAP_PERFMON; then
+    echo "Warning: build environment does not support CAP_PERFMON, you must setuid root on this platform" >&2
+    exit 0
+fi
+
+"$setcap" cap_perfmon=+ep "$exe_fullpath"

src/meson.build

@@ -257,14 +257,24 @@ endif
 
 if get_option('mango_intel_stats').enabled() and is_unixy
   drm_dep = cc.find_library('libdrm')
-  warning('The mango_intel_stats binary must have CAP_PERFMON or setuid to be useful')
-  warning('You must do this manually post-install: setcap cap_perfmon=+ep /path/to/mango_intel_stats')
+  setcap = find_program('setcap', '/usr/sbin/setcap', '/sbin/setcap', required : false)
   mangointel = executable(
     'mango_intel_stats',
     files('app/intel.cpp'),
     install : true,
     dependencies : [drm_dep, json_dep],
   )
+  if setcap.found()
+    meson.add_install_script('app/setcap-helper.sh',
+      setcap.full_path(),
+      get_option('bindir'),
+      mangointel.name(),
+      )
+  else
+    warning('The mango_intel_stats binary must have CAP_PERFMON or setuid to be useful')
+    warning('Install setcap to have CAP_PERFMON set automatically, or')
+    warning('You must do this manually post-install: setcap cap_perfmon=+ep /path/to/mango_intel_stats')
+  endif
 endif
 
 if get_option('mangoapp_layer')