It seems thunar doesn't like xfce4-panel-4.9.0.
I'm getting a build failed with

Making all in thunar-tpa
make[3]: Entering directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins/thunar-tpa'
make  all-am
make[4]: Entering directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins/thunar-tpa'
  CC     libthunar_tpa_la-thunar-tpa.lo
thunar-tpa.c:55:49: error: unknown type name 'XfcePanelTypeModule'
thunar-tpa.c:59:74: error: missing binary operator before token "("
thunar-tpa.c:108:3: error: unknown type name 'XfcePanelPluginClass'
thunar-tpa.c:113:19: error: field '__parent__' has incomplete type
thunar-tpa.c:141:38: error: unknown type name 'thunar_tpa'
thunar-tpa.c:157:74: error: missing binary operator before token "("
thunar-tpa.c: In function 'thunar_tpa_init':
thunar-tpa.c:182:17: warning: assignment makes pointer from integer without a cast [enabled by default]
thunar-tpa.c: In function 'thunar_tpa_finalize':
thunar-tpa.c:235:5: error: 'thunar_tpa_parent_class' undeclared (first use in this function)
thunar-tpa.c:235:5: note: each undeclared identifier is reported only once for each function it appears in
thunar-tpa.c: In function 'thunar_tpa_construct':
thunar-tpa.c:245:74: error: missing binary operator before token "("
thunar-tpa.c: At top level:
thunar-tpa.c:259:74: error: missing binary operator before token "("
make[4]: *** [libthunar_tpa_la-thunar-tpa.lo] Error 1
make[4]: Leaving directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins/thunar-tpa'
make[3]: *** [all] Error 2
make[3]: Leaving directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins/thunar-tpa'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/tmp/xfce-build-dir/Thunar-1.3.0'
make: *** [all] Error 2

I'm using Slackware-CuMaking all in thunar-tpa
make[3]: Entering directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins/thunar-tpa'
make  all-am
make[4]: Entering directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins/thunar-tpa'
  CC     libthunar_tpa_la-thunar-tpa.lo
thunar-tpa.c:55:49: error: unknown type name 'XfcePanelTypeModule'
thunar-tpa.c:59:74: error: missing binary operator before token "("
thunar-tpa.c:108:3: error: unknown type name 'XfcePanelPluginClass'
thunar-tpa.c:113:19: error: field '__parent__' has incomplete type
thunar-tpa.c:141:38: error: unknown type name 'thunar_tpa'
thunar-tpa.c:157:74: error: missing binary operator before token "("
thunar-tpa.c: In function 'thunar_tpa_init':
thunar-tpa.c:182:17: warning: assignment makes pointer from integer without a cast [enabled by default]
thunar-tpa.c: In function 'thunar_tpa_finalize':
thunar-tpa.c:235:5: error: 'thunar_tpa_parent_class' undeclared (first use in this function)
thunar-tpa.c:235:5: note: each undeclared identifier is reported only once for each function it appears in
thunar-tpa.c: In function 'thunar_tpa_construct':
thunar-tpa.c:245:74: error: missing binary operator before token "("
thunar-tpa.c: At top level:
thunar-tpa.c:259:74: error: missing binary operator before token "("
make[4]: *** [libthunar_tpa_la-thunar-tpa.lo] Error 1
make[4]: Leaving directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins/thunar-tpa'
make[3]: *** [all] Error 2
make[3]: Leaving directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins/thunar-tpa'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/tmp/xfce-build-dir/Thunar-1.3.0/plugins'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/tmp/xfce-build-dir/Thunar-1.3.0'
make: *** [all] Error 2

I'm using Slackware-Current, tried Thunar-1.2.3, also xfce4-panel-4.8.x same error.

Thunar built successfully though with slackware stock xfce4-panel-4.6.4.