Do not regenerate importlib.h and importlib_external.h when cross-com…

…piling

They are already present in source releases, so it is possible to ignore
the regeneration step. We do that only when cross-compiling, because it
is not possible to build for both the host and the target with a single
pass of CMake.

cmake/libpython/CMakeLists.txt

@@ -455,7 +455,7 @@ if(WIN32 AND IS_PY3)
 endif()
 
 set(LIBPYTHON_FROZEN_SOURCES )
-if(IS_PY3)
+if(IS_PY3 AND NOT CMAKE_CROSSCOMPILING)
 
 # Build _freeze_importlib executable
 add_executable(_freeze_importlib
@@ -512,6 +512,11 @@ endif()
 # the frozen sources.
 add_custom_target(freeze_modules DEPENDS ${LIBPYTHON_FROZEN_SOURCES})
 
+elseif(IS_PY3 AND CMAKE_CROSSCOMPILING)
+
+# KLUDGE: suppose importlib.h and importlib_external.h are already present.
+add_custom_target(freeze_modules)
+
 endif()
 
 if(PY_VERSION VERSION_LESS "3.8")