Support zlib in non-standard path on Unix

Found by: Michael Osipov
Fixes: SERF-180

* SConstruct: Add ZLIB path in non-win32 case


git-svn-id: https://svn.apache.org/repos/asf/serf/trunk@1761040 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/SConstruct b/SConstruct
index 41d2983..b6f22fc 100644
--- a/SConstruct
+++ b/SConstruct
@@ -400,6 +400,9 @@
     apr_libs = ''
     apu_libs = ''
 
+  env.Append(CPPPATH=['$ZLIB/include'])
+  env.Append(LIBPATH=['$ZLIB/lib'])
+
   env.Append(CPPPATH=['$OPENSSL/include'])
   env.Append(LIBPATH=['$OPENSSL/lib'])