build: Tweak default pkg-config search paths

Change-Id: Id9b656104b00fd18c9302165b0eeeffd17f6c87a
Fixes: #2711
diff --git a/wscript b/wscript
index 1af218a..60bdb99 100644
--- a/wscript
+++ b/wscript
@@ -70,9 +70,7 @@
     conf.find_program('bash', var='BASH')
 
     if 'PKG_CONFIG_PATH' not in os.environ:
-        os.environ['PKG_CONFIG_PATH'] = ':'.join([
-            '/usr/local/lib/pkgconfig',
-            '/opt/local/lib/pkgconfig'])
+        os.environ['PKG_CONFIG_PATH'] = Utils.subst_vars('${LIBDIR}/pkgconfig', conf.env)
     conf.check_cfg(package='libndn-cxx', args=['--cflags', '--libs'],
                    uselib_store='NDN_CXX', mandatory=True)