virt-wireshark.m4: Defer $(prefix) substitution

The autoconf manual says we shouldn't be using this variable (and
some others) outside of Makefiles as it prevents users providing
their own prefix at the installation phase.

https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Installation-Directory-Variables.html

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

m4/virt-wireshark.m4

@@ -38,7 +38,7 @@ AC_DEFUN([LIBVIRT_CHECK_WIRESHARK],[
         if test "x$ws_prefix" = "x" ; then
           ws_prefix="/usr";
         fi
-        plugindir="${prefix}${plugindir#$ws_prefix}"
+        plugindir="${plugindir#$ws_prefix}"
       fi
     elif test "x$with_ws_plugindir" = "xno" || test "x$with_ws_plugindir" = "xyes"; then
       AC_MSG_ERROR([ws-plugindir must be used only with valid path])

tools/Makefile.am

@@ -398,7 +398,7 @@ EXTRA_DIST += \
 
 if WITH_WIRESHARK_DISSECTOR
 
-ws_plugindir = $(plugindir)
+ws_plugindir = $(prefix)$(plugindir)
 ws_plugin_LTLIBRARIES = wireshark/src/libvirt.la
 wireshark_src_libvirt_la_CPPFLAGS = \
 	-I wireshark/src $(WIRESHARK_DISSECTOR_CFLAGS)