fix ill-defined variable EVALHYD_TESTING_OS

tests/CMakeLists.txt

@@ -8,25 +8,28 @@ find_package(GTest REQUIRED)
 # ------------------------------------------------------------------------------
 # build
 # ------------------------------------------------------------------------------
+add_executable(
+        evalhyd_tests
+        test_determinist.cpp
+        test_probabilist.cpp
+        test_uncertainty.cpp
+)
+
 OPTION(EVALHYD_TESTING_OS "OS system used to run tests")
 
 if(CMAKE_HOST_APPLE)
-    set(EVALHYD_TESTING_OS "MACOS")
+    target_compile_definitions(evalhyd_tests PRIVATE EVALHYD_TESTING_OS MACOS)
+    message(STATUS "Found supported OS to run tests: APPLE")
 elseif(CMAKE_HOST_WIN32)
-    set(EVALHYD_TESTING_OS "WINDOWS")
+    target_compile_definitions(evalhyd_tests PRIVATE EVALHYD_TESTING_OS WINDOWS)
+    message(STATUS "Found supported OS to run tests: WIN32")
 elseif(CMAKE_HOST_UNIX)
-    set(EVALHYD_TESTING_OS "LINUX")
+    target_compile_definitions(evalhyd_tests PRIVATE EVALHYD_TESTING_OS LINUX)
+    message(STATUS "Found supported OS to run tests: UNIX")
 else()
     message(SEND_ERROR "OS not supported to run tests")
 endif()
 
-add_executable(
-        evalhyd_tests
-        test_determinist.cpp
-        test_probabilist.cpp
-        test_uncertainty.cpp
-)
-
 set_target_properties(
         evalhyd_tests
         PROPERTIES