PHP: use a macro to specify extension src dir

config.m4

@@ -5,9 +5,9 @@ if test "$PHP_GRPC" != "no"; then
   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"