CMake: Produce correct plugindir for pkg-config

CMake: Produce correct plugindir for pkg-config

So far the plugindir variable in the pkg-config file is set like
this:

  set( plugindir "\${libdir}/${PLUGIN_INSTALL_DIR}" )

where libdir is set to:

  set( libdir "\${exec_prefix}/${CMAKE_INSTALL_LIBDIR}" )

However, PLUGIN_INSTALL_DIR already contains
CMAKE_INSTALL_LIBDIR:

  set(PLUGIN_INSTALL_DIR "${CMAKE_INSTALL_LIBDIR}/\
  ${CPACK_PACKAGE_NAME}/plugins/${CPACK_PACKAGE_VERSION}")

Therefore, if the pkg-config file is generated by cmake, the
resulting plugindir points to:

   plugindir=${libdir}/lib/wireshark/plugins/2.1.0
   libdir=${exec_prefix}/lib

This is obviously wrong as after variable substitution the
resulting path is /usr/lib/lib/wireshark/plugins/...

Change-Id: I448991284d8948434311b1c0828828fd93d0baf8
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-on: https://code.wireshark.org/review/13258
Tested-by: João Valverde <j@v6e.pt>
Reviewed-by: João Valverde <j@v6e.pt>