PHP: use a macro to specify extension src dir
diff --git a/config.m4 b/config.m4
index 12b1057..85549e5 100644
--- a/config.m4
+++ b/config.m4
@@ -5,9 +5,9 @@
   dnl Write more examples of tests here...
 
   dnl # --with-grpc -> add include path
-  PHP_ADD_INCLUDE(../../grpc/include)
-  PHP_ADD_INCLUDE(../../grpc/src/php/ext/grpc)
-  PHP_ADD_INCLUDE(../../grpc/third_party/boringssl/include)
+  PHP_ADD_INCLUDE(PHP_EXT_SRCDIR()/include)
+  PHP_ADD_INCLUDE(PHP_EXT_SRCDIR()/src/php/ext/grpc)
+  PHP_ADD_INCLUDE(PHP_EXT_SRCDIR()/third_party/boringssl/include)
 
   LIBS="-lpthread $LIBS"