seccomp: Get actual errno value from failed seccomp functions

Upon failure, a libseccomp API returns actual errno value very
rarely. Fortunately, after its commit 34bf78ab (contained in
2.5.0 release), the SCMP_FLTATR_API_SYSRAWRC attribute can be set
which makes subsequent APIs return true errno on failure.

This is especially critical when seccomp_load() fails, because
generic -ECANCELED says nothing.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
diff --git a/meson.build b/meson.build
index b686dfe..5f114c8 100644
--- a/meson.build
+++ b/meson.build
@@ -636,10 +636,16 @@
 endif
 
 seccomp = not_found
+seccomp_has_sysrawrc = false
 if not get_option('seccomp').auto() or have_system or have_tools
   seccomp = dependency('libseccomp', version: '>=2.3.0',
                        required: get_option('seccomp'),
                        method: 'pkg-config', kwargs: static_kwargs)
+  if seccomp.found()
+    seccomp_has_sysrawrc = cc.has_header_symbol('seccomp.h',
+                                                'SCMP_FLTATR_API_SYSRAWRC',
+                                                dependencies: seccomp)
+  endif
 endif
 
 libcap_ng = not_found
@@ -1849,6 +1855,9 @@
 config_host_data.set('CONFIG_SDL', sdl.found())
 config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
 config_host_data.set('CONFIG_SECCOMP', seccomp.found())
+if seccomp.found()
+  config_host_data.set('CONFIG_SECCOMP_SYSRAWRC', seccomp_has_sysrawrc)
+endif
 config_host_data.set('CONFIG_SNAPPY', snappy.found())
 config_host_data.set('CONFIG_TPM', have_tpm)
 config_host_data.set('CONFIG_USB_LIBUSB', libusb.found())