[CMake] problem: No test configuration file found!

[Robert Ramey]

I'm working through my first CMake/CTest project.

My directory structure looks like

safe_numerics/
    CMakeLists.txt // #1
    build/ // out of source build directory
    include/
    examples/
    doc/
    tests/
        CMakeLists.txt // #2


#1 looks like
cmake_minimum_required(VERSION 2.6)
project(safe_numerics)
if(MSVC)
add_definitions(
    /Gy
    /wd4244 # converting 'std::streamsize' in 'size_t'
    /D_CRT_SECURE_NO_WARNINGS
    /D_SCL_SECURE_NO_WARNINGS
)
endif(MSVC)
set(CMAKE_VERBOSE_MAKEFILE on)
set(CMAKE_BUILD_TYPE Debug)
message("BOOST_ROOT = $ENV{BOOST_ROOT}" )
include_directories($ENV{BOOST_ROOT})
# walk the tests directory to run tests
add_subdirectory(tests)

#2 looks like
ENABLE_TESTING()
add_executable(test_subtract
    test_subtract.cpp
    test_subtract1.cpp
    test_subtract2.cpp
    test_subtract3.cpp
)
add_test(
    test_subtract
    test_subtract
)

Here is what I do

cd build
cmake ..
$ ls -1
ALL_BUILD.vcproj
CMakeCache.txt
CMakeFiles
ZERO_CHECK.vcproj
cmake_install.cmake
safe_numerics.ncb
safe_numerics.sln
tests


when I bring up safe_numerics.sln, I do see the test_subtract project, it 
builds and executes.

BUT when I try to run CTest I get (among other stuff)

$ctest
*********************************
No test configuration file found!
*********************************
...

Things I've tried.

I eliminate the add_subdirectory and put all the sommand into file #1 . 
This did in fact build RUN_TESTS.vcproj.  But I can't see what I did wroing 
and what the "right way" to fix it is.

I'm lost as to what's wrong with this.  Any help appreciated.

Robert Ramey