Unit tests using Automake

I am working in a project with other people in the team using GNU autotools. In the project we are using unit test for each non trivial C++ class. I found out that there is support for unit testing. For that I am using this structure:

 ./
   + tests/
     + Makefile.am
     + classA_test.cc
     ....
     + classB_test.cc
   + src/
   + lib/
   + Makefile.am 

The problem comes since my main Makefile.am is using subdir-objects options --note that I am not using recursive makefile for the source files--, I cannot export my variables --such as, AM_CPPFLAGS-- to the other Makefile. So far I made it work using:

  $ make check      

but I keep getting problems for the paths and the options when I do

  $ make distcheck

So my questions is, how is the standard way to deal with unit tests?