Set BORNAGAIN_USE_PYTHON3 to ON when python3 is the python found by cmake

cmake/bornagain/modules/SearchInstalledSoftware.cmake

@@ -60,6 +60,11 @@ if(BORNAGAIN_PYTHON OR BORNAGAIN_GUI)
     endif()
     message(STATUS "Found Python libraries version ${PYTHONLIBS_VERSION_STRING} at ${PYTHON_LIBRARIES}; includes at ${PYTHON_INCLUDE_DIRS}")
 
+    if(PYTHONLIBS_VERSION_STRING MATCHES "^3.*$")
+        set(BORNAGAIN_USE_PYTHON3 ON)
+        message(STATUS "Python 3 will be used. Setting BORNAGAIN_USE_PYTHON3 to ${BORNAGAIN_USE_PYTHON3}.")
+    endif()
+
     if(NOT WIN32)
         include(ValidatePythonInstallation)
         ValidatePythonInstallation()