configure, meson: convert libtasn1 detection to meson

Make it depend on gnutls too, since it is only used as part of gnutls
tests.

Reviewed-by: Richard Henderson <richard.henderson@liaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/meson.build b/meson.build
index 286b37a..d4ce2ca 100644
--- a/meson.build
+++ b/meson.build
@@ -985,9 +985,10 @@
   libdaxctl = declare_dependency(link_args: config_host['LIBDAXCTL_LIBS'].split())
 endif
 tasn1 = not_found
-if 'CONFIG_TASN1' in config_host
-  tasn1 = declare_dependency(compile_args: config_host['TASN1_CFLAGS'].split(),
-                             link_args: config_host['TASN1_LIBS'].split())
+if gnutls.found()
+  tasn1 = dependency('libtasn1',
+                     method: 'pkg-config',
+                     kwargs: static_kwargs)
 endif
 keyutils = dependency('libkeyutils', required: false,
                       method: 'pkg-config', kwargs: static_kwargs)
@@ -2727,7 +2728,7 @@
 summary_info += {'VTE support':       config_host.has_key('CONFIG_VTE')}
 # TODO: add back version
 summary_info += {'slirp support':     slirp_opt == 'disabled' ? false : slirp_opt}
-summary_info += {'libtasn1':          config_host.has_key('CONFIG_TASN1')}
+summary_info += {'libtasn1':          tasn1.found()}
 summary_info += {'PAM':               config_host.has_key('CONFIG_AUTH_PAM')}
 summary_info += {'iconv support':     iconv.found()}
 summary_info += {'curses support':    curses.found()}