build: Fix and improve generated pkgconfig files

Includes:
- ignore '-l' option for absolute paths
- remove duplicate include directories

build-support/custom-modules/ns3-cmake-package.cmake

@@ -27,10 +27,15 @@ function(build_required_and_libs_lists module_name visibility libraries
       set(required_modules_list
           "${required_modules_list} ns3-${required_module_name}"
       )
+      set(lib_real_name "-l${lib_real_name}")
     else()
-      set(lib_real_name ${lib})
+      if(IS_ABSOLUTE ${lib})
+        set(lib_real_name ${lib})
+      else()
+        set(lib_real_name "-l${lib}")
+      endif()
     endif()
-    set(linked_libs_list "${linked_libs_list} -l${lib_real_name}")
+    set(linked_libs_list "${linked_libs_list} ${lib_real_name}")
   endforeach()
   set(pkgconfig_${visibility}_libs ${linked_libs_list} PARENT_SCOPE)
   set(pkgconfig_${visibility}_required ${required_modules_list} PARENT_SCOPE)

build-support/macros-and-definitions.cmake

@@ -2086,6 +2086,7 @@ endfunction()
 function(get_target_includes target output)
   set(include_directories)
   get_target_property(include_dirs ${target} INCLUDE_DIRECTORIES)
+  list(REMOVE_DUPLICATES include_dirs)
   foreach(include_dir ${include_dirs})
     if(include_dir MATCHES "<")
       # Skip CMake build and install interface includes