build: replace CMARK_TESTS with BUILD_TESTING

`BUILD_TESTING` is the CMake sanctioned option for the control of tests
(c.f. https://cmake.org/cmake/help/latest/module/CTest.html).  Use the
standard option to control the build of test targets.

CMakeLists.txt

@@ -8,7 +8,6 @@ if("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_BINARY_DIR}")
     message(FATAL_ERROR "Do not build in-source.\nPlease remove CMakeCache.txt and the CMakeFiles/ directory.\nThen: mkdir build ; cd build ; cmake .. ; make")
 endif()
 
-option(CMARK_TESTS "Build cmark tests and enable testing" ON)
 option(CMARK_STATIC "Build static libcmark library" ON)
 option(CMARK_SHARED "Build shared libcmark library" ON)
 option(CMARK_LIB_FUZZER "Build libFuzzer fuzzing harness" OFF)
@@ -60,16 +59,18 @@ function(cmark_add_compile_options target)
 endfunction()
 
 add_subdirectory(src)
-if(CMARK_TESTS AND (CMARK_SHARED OR CMARK_STATIC))
-  add_subdirectory(api_test)
+if(BUILD_TESTING)
 endif()
 # TODO(compnerd) should this be enabled for MinGW, which sets CMAKE_SYSTEM_NAME
 # to Windows, but defines `MINGW`.
 if(NOT CMAKE_SYSTEM_NAME STREQUAL Windows)
   add_subdirectory(man)
 endif()
-if(CMARK_TESTS)
+if(BUILD_TESTING)
   enable_testing()
+  if(CMARK_SHARED OR CMARK_STATIC)
+    add_subdirectory(api_test)
+  endif()
   add_subdirectory(test testdir)
 endif()
 

api_test/CMakeLists.txt

@@ -10,3 +10,6 @@ if(CMARK_SHARED)
 else()
   target_link_libraries(api_test cmark_static)
 endif()
+
+add_test(NAME api_test COMMAND api_test)
+

test/CMakeLists.txt

@@ -19,10 +19,6 @@ else()
     IMPORTED_LOCATION ${PYTHON_EXECUTABLE})
 endif()
 
-if (CMARK_SHARED OR CMARK_STATIC)
-  add_test(NAME api_test COMMAND api_test)
-endif()
-
 if (WIN32)
   file(TO_NATIVE_PATH ${CMAKE_BINARY_DIR}/src WIN_DLL_DIR)
   set(NEWPATH "${WIN_DLL_DIR};$ENV{PATH}")
@@ -82,9 +78,7 @@ IF (Python_Interpreter_FOUND)
 
 ELSE(Python_Interpreter_FOUND)
 
-  message("\n*** A python 3 interpreter is required to run the spec tests.\n")
-  add_test(skipping_spectests
-    echo "Skipping spec tests, because no python 3 interpreter is available.")
+  message(WARNING "A Python 3 Interpreter is required to run the spec tests")
 
 ENDIF(Python_Interpreter_FOUND)