Avoid deprecation warning for -py3 in swig >= 4.1

CMakeLists.txt

@@ -278,7 +278,6 @@ endif()
 # SWIG
 find_package (SWIG 3.0.12 REQUIRED)
 include (${SWIG_USE_FILE})
-string(REGEX MATCH "^[0-9]+[.]" SWIG_MAJOR ${SWIG_VERSION})
 
 # Find Guile and determine which version we are using.
 # Look for guile versions in this order: 3.0 > 2.2 > 2.0

common/cmake_modules/GncAddSwigCommand.cmake

@@ -69,10 +69,15 @@ macro (gnc_add_swig_python_command _target _out_var _py_out_var _output _py_outp
     set(${_py_out_var} ${py_outfile}) # This variable is set for convenience to use in the calling CMakeLists.txt
 
     set (DEFAULT_SWIG_PYTHON_FLAGS
-        -python -py3
+        -python
         -Wall -Werror
         ${SWIG_ARGS}
         )
+    if (${SWIG_VERSION} VERSION_LESS 4.1)
+        set (DEFAULT_SWIG_PYTHON_FLAGS
+            ${SWIG_ARGS} -py3
+            )
+    endif()
     set (DEFAULT_SWIG_PYTHON_C_INCLUDES
          ${GLIB2_INCLUDE_DIRS}
          ${CMAKE_SOURCE_DIR}/common