FindPkgConfig: correct library search path for imported targets.

`-L` in LDFLAGS doesn't mean that standard search paths should be
excluded. Example:

    $ pkg-config --libs libmutter-2
    -L/usr/lib/mutter -lmutter-2 -lgtk-3 -lgdk-3 -lmutter-clutter-2 -lcairo-gobject -latk-1.0 -lpangocairo-1.0 -lpango-1.0 -lfribidi -ljson-glib-1.0 -lgio-2.0 -lwayland-egl -lwayland-client -lXtst -lXi -lmutter-cogl-2 -lcairo
-lgmodule-2.0 -pthread -lgdk_pixbuf-2.0 -lgobject-2.0 -lglib-2.0 -lgbm -ldrm -lwayland-server -lEGL -lXext -lXdamage -lXfixes -lXcomposite -lXrandr -lX11

Most of these libraries are located at standard paths, not in
`/usr/lib/mutter`.

Fixes: #17954

Modules/FindPkgConfig.cmake

@@ -200,9 +200,7 @@ function(_pkg_create_imp_target _prefix _no_cmake_path _no_cmake_environment_pat
   unset(_search_paths)
   foreach (flag IN LISTS ${_prefix}_LDFLAGS)
     if (flag MATCHES "^-L(.*)")
-      # only look into the given paths from now on
       list(APPEND _search_paths ${CMAKE_MATCH_1})
-      set(_find_opts HINTS ${_search_paths} NO_DEFAULT_PATH)
       continue()
     endif()
     if (flag MATCHES "^-l(.*)")
@@ -211,6 +209,12 @@ function(_pkg_create_imp_target _prefix _no_cmake_path _no_cmake_environment_pat
       continue()
     endif()
 
+    if(_search_paths)
+        # Firstly search in -L paths
+        find_library(pkgcfg_lib_${_prefix}_${_pkg_search}
+                     NAMES ${_pkg_search}
+                     HINTS ${_search_paths} NO_DEFAULT_PATH)
+    endif()
     find_library(pkgcfg_lib_${_prefix}_${_pkg_search}
                  NAMES ${_pkg_search}
                  ${_find_opts})