CMake: Avoid executing tests during the build phase

GoogleTest: `gtest_discover_tests` discovers tests by asking the _compiled test
executable_ to enumerate its tests. `PRE_TEST` delays test discovery until just
prior to test execution; this avoids calling the executables during the build
phase and hence, error messages like "Error running test executable".

Related to issue #133

CMakeLists.txt

@@ -37,6 +37,11 @@ if(NOT CMAKE_BUILD_TYPE)
 endif()
 message("    CMAKE_BUILD_TYPE: ${CMAKE_BUILD_TYPE}")
 
+# GoogleTest: `gtest_discover_tests` discovers tests by asking the _compiled test executable_
+# to enumerate its tests. `PRE_TEST` delays test discovery until just prior to test execution;
+# this avoids calling the executables during the build phase.
+set(CMAKE_GTEST_DISCOVER_TESTS_DISCOVERY_MODE PRE_TEST)
+
 include(CTest) # equivalent to "enable_testing() ???
 add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND} -LE Fullcheck)
    # => 'make check' is an alias for 'ctest'