[PROTOBUF-3404] add --with-zlib=PATH to configure.ac script

configure.ac

@@ -52,6 +52,16 @@ AC_ARG_WITH([zlib],
     [include classes for streaming compressed data in and out @<:@default=check@:>@])],
   [],[with_zlib=check])
 
+AC_ARG_WITH([zlib-include],
+  [AS_HELP_STRING([--with-zlib-include=PATH],
+    [zlib include directory])],
+  [CPPFLAGS="-I$withval $CPPFLAGS"])
+
+AC_ARG_WITH([zlib-lib],
+  [AS_HELP_STRING([--with-zlib-lib=PATH],
+    [zlib lib directory])],
+  [LDFLAGS="-L$withval $LDFLAGS"])
+
 AC_ARG_WITH([protoc],
   [AS_HELP_STRING([--with-protoc=COMMAND],
     [use the given protoc command instead of building a new one when building tests (useful for cross-compiling)])],