As the library is only built in the master port, and therfore missing in the
build directory for the slave ports, find it via pkg-config and link against it,
when building a slave.

--- bindings/perl/src/CMakeLists.txt.orig	2018-06-03 17:39:41 UTC
+++ bindings/perl/src/CMakeLists.txt
@@ -4,7 +4,7 @@ include_directories( ${PERL_INCLUDE_PATH} ${CMAKE_SOUR
 add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/Libproxy.c
                    COMMAND ${PERL_EXECUTABLE} ARGS ${CMAKE_CURRENT_SOURCE_DIR}/doxsubpp.pl
                            ${PERL_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/Libproxy.xs ${CMAKE_CURRENT_BINARY_DIR}/Libproxy.c
-                   DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/Libproxy.xs libproxy
+                   DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/Libproxy.xs
                    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 
 set(Libproxy_LIB_SRCS Libproxy.c)
@@ -12,7 +12,7 @@ set(Libproxy_LIB_SRCS Libproxy.c)
 set(LIBRARY_OUTPUT_PATH ${CMAKE_BINARY_DIR}/perl/blib/arch/auto/Net)
 add_library(PLlibproxy SHARED ${Libproxy_LIB_SRCS})
 
-set(PLlibproxy_LIB_DEPENDENCIES libproxy pthread)
+set(PLlibproxy_LIB_DEPENDENCIES PkgConfig::LIBPROXY pthread)
 if(PERL_LINK_LIBPERL)
   set(PLlibproxy_LIB_DEPENDENCIES ${PERL_LIBRARY} ${PLlibproxy_LIB_DEPENDENCIES})
 endif()