schemagen: move test configuration into contrib/schemagen/CMakeLists.txt

Addresses review feedback: keep the schemagen-specific CMake rules
close to the tool instead of inline in the top-level CMakeLists.txt.
The subdirectory file is added from the root and guarded by the same
Python3 availability check that was already in use.

CMakeLists.txt

@@ -170,37 +170,7 @@ if(Python3_Interpreter_FOUND)
                    ${CMAKE_CURRENT_SOURCE_DIR}/test_python_bindings.py)
 endif()
 
-# schemagen tests
+add_subdirectory(contrib/schemagen)
-add_test(
-  NAME schemagen_python_tests
-  COMMAND python3 contrib/schemagen/test_schemagen.py
-  WORKING_DIRECTORY ${CMAKE_SOURCE_DIR})
-
-set(SCHEMAGEN_DIR ${CMAKE_SOURCE_DIR}/contrib/schemagen)
-set(SCHEMAGEN_SCRIPT ${SCHEMAGEN_DIR}/weaseljson_schemagen.py)
-set(EXAMPLE_SCHEMA ${SCHEMAGEN_DIR}/example.schema.json)
-set(GEN_H ${CMAKE_BINARY_DIR}/gen.h)
-
-add_custom_command(
-  OUTPUT ${GEN_H}
-  COMMAND python3 ${SCHEMAGEN_SCRIPT} ${EXAMPLE_SCHEMA} -o ${GEN_H} --namespace
-          weasel_schema
-  DEPENDS ${SCHEMAGEN_SCRIPT} ${EXAMPLE_SCHEMA}
-  COMMENT "Generating gen.h from example.schema.json")
-
-add_custom_target(schemagen_gen_h DEPENDS ${GEN_H})
-
-add_executable(schemagen_example ${SCHEMAGEN_DIR}/test_gen.cpp)
-target_include_directories(schemagen_example PRIVATE include
-                                                     ${CMAKE_BINARY_DIR})
-target_link_libraries(schemagen_example PRIVATE ${PROJECT_NAME})
-target_compile_options(schemagen_example PRIVATE -Wno-switch-enum)
-add_dependencies(schemagen_example schemagen_gen_h)
-
-add_test(
-  NAME schemagen_example
-  COMMAND schemagen_example
-  WORKING_DIRECTORY ${CMAKE_BINARY_DIR})
 
 include(CMakePushCheckState)
 include(CheckCXXCompilerFlag)

contrib/schemagen/CMakeLists.txt

@@ -0,0 +1,35 @@
+# Tests for contrib/schemagen
+
+if(NOT Python3_Interpreter_FOUND)
+  return()
+endif()
+
+add_test(
+  NAME schemagen_python_tests
+  COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test_schemagen.py
+  WORKING_DIRECTORY ${CMAKE_SOURCE_DIR})
+
+set(SCHEMAGEN_SCRIPT ${CMAKE_CURRENT_SOURCE_DIR}/weaseljson_schemagen.py)
+set(EXAMPLE_SCHEMA ${CMAKE_CURRENT_SOURCE_DIR}/example.schema.json)
+set(GEN_H ${CMAKE_CURRENT_BINARY_DIR}/gen.h)
+
+add_custom_command(
+  OUTPUT ${GEN_H}
+  COMMAND ${Python3_EXECUTABLE} ${SCHEMAGEN_SCRIPT} ${EXAMPLE_SCHEMA} -o
+          ${GEN_H} --namespace weasel_schema
+  DEPENDS ${SCHEMAGEN_SCRIPT} ${EXAMPLE_SCHEMA}
+  COMMENT "Generating gen.h from example.schema.json")
+
+add_custom_target(schemagen_gen_h DEPENDS ${GEN_H})
+
+add_executable(schemagen_example ${CMAKE_CURRENT_SOURCE_DIR}/test_gen.cpp)
+target_include_directories(schemagen_example
+                           PRIVATE include ${CMAKE_CURRENT_BINARY_DIR})
+target_link_libraries(schemagen_example PRIVATE ${PROJECT_NAME})
+target_compile_options(schemagen_example PRIVATE -Wno-switch-enum)
+add_dependencies(schemagen_example schemagen_gen_h)
+
+add_test(
+  NAME schemagen_example
+  COMMAND schemagen_example
+  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})