build: Update waf to version 1.8.8-dev (with custom patches)

https://github.com/cawka/waf/tree/8f714a523d3b8c66f2949aaa8db91d6f7be55e55

This patch also embeds the default configuration for `PKG_CONFIG_PATH` variable.

Change-Id: I8949c1e068fed12b8c6d291ed3e5e1fe85df042e
Refs: #2711
diff --git a/wscript b/wscript
index 8aaea63..683419c 100644
--- a/wscript
+++ b/wscript
@@ -24,10 +24,8 @@
                'boost', 'default-compiler-flags', 'doxygen', 'sphinx_build',
                'sqlite3', 'pch', 'coverage'])
 
-    if not os.environ.has_key('PKG_CONFIG_PATH'):
-        os.environ['PKG_CONFIG_PATH'] = ':'.join([
-            '/usr/local/lib/pkgconfig',
-            '/opt/local/lib/pkgconfig'])
+    if 'PKG_CONFIG_PATH' not in os.environ:
+        os.environ['PKG_CONFIG_PATH'] = Utils.subst_vars('${LIBDIR}/pkgconfig', conf.env)
 
     conf.check_cfg(package='liblog4cxx', args=['--cflags', '--libs'],
                   uselib_store='LOG4CXX', mandatory=True)