diff --git a/config.m4 b/config.m4
index ab3c4a3..bb8268f 100644
--- a/config.m4
+++ b/config.m4
@@ -1,19 +1,25 @@
 PHP_ARG_ENABLE(protocolbuffers, Whether to enable the "protocolbuffers" extension,
-    [ --enable-protocolbuffers  Enable "protocolbuffers" extension support])
+    [ --enable-protocolbuffers[=DIR]  Enable "protocolbuffers" extension support])
 
 PHP_ARG_ENABLE(protocolbuffers-debug, for protocolbuffers debug support,
     [ --enable-protocolbuffers-debug       Enable enable protocolbuffers debug support], no, no)
 
 if test $PHP_PROTOCOLBUFFERS != "no"; then
+    if test -r "$PHP_PROTOCOLBUFFERS/include/google/protobuf/message.h"; then
+       PROTOBUF_DIR="$PHP_PROTOCOLBUFFERS"
+    fi
+    if test -z "$PROTOBUF_DIR"; then
+       AC_MSG_ERROR(Protobuf headers not found)
+    fi
+
     if test "$PHP_PROTOCOLBUFFERS_DEBUG" != "no"; then
         CFLAGS="$CFLAGS -Wall -g -ggdb -O0 -DPHP_PROTOCOLBUFFERS_DEBUG=1"
         AC_DEFINE(PHP_PROTOCOLBUFFERS_DEBUG, 1, [Enable protocolbuffers debug support])
-    else
-        //CFLAGS="$CFLAGS -Wall -g -ggdb -O0 -Wunused-variable -Wpointer-sign -Wimplicit-function-declaration -Winline -Wunused-macros -Wredundant-decls -Wstrict-aliasing=2 -Wswitch-enum -Wdeclaration-after-statement"
     fi
 
     PHP_PROTOBUF_SRCS="protocolbuffers.c descriptor.c descriptor_builder.c field_descriptor.c message.c message_options.c enum.c enum_descriptor.c enum_value_descriptor.c enum_descriptor_builder.c php_message_options.c unknown_field.c unknown_field_set.c helper.c extension_registry.c core.c serializer.c json_serializer.c"
 
+    PHP_ADD_INCLUDE($PROTOBUF_DIR/include)
     PHP_SUBST(PROTOCOLBUFFERS_SHARED_LIBADD)
     PHP_NEW_EXTENSION(protocolbuffers, $PHP_PROTOBUF_SRCS, $ext_shared)
     PHP_ADD_EXTENSION_DEP(protocolbuffers, spl, true)