If Catch 2 and Catch 1 is not found, then abort.

tests.CMakeLists.txt

@@ -3,8 +3,13 @@ if(WITH_TESTS)
     if(Catch2_FOUND)
         set(MYCATCH, Catch2::Catch2)
     else()
-        message(WARNING "Catch2 not found. Assuming Catch 1.x is installed.")
-        set(MYCATCH, "")
+        if(EXISTS "/usr/include/catch.hpp")
+            message(STATUS "Catch 1.x found.")
+            set(MYCATCH, "")
+        else()
+            message(FATAL_ERROR
+                    "Neither Catch 2.x nor Catch 1.x could be found.")
+        endif()
     endif()
     include(CTest)