py/mkrules.mk: Reset USER_C_MODULES when building mpy-cross dependency.

When a port automatically compiles `mpy-cross`, if `USER_C_MODULES` is
provided by the user on the command line then it is also applied to the
`mpy-cross` build.  That can lead to build errors if the path is relative
and not found when building `mpy-cross`.

Fix that by explicitly resetting `USER_C_MODULES` when invoking the
`mpy-cross` build.

Signed-off-by: Andrew Leech <[email protected]>

Author: pi-anl

py/mkrules.cmake

@@ -211,7 +211,7 @@ if(MICROPY_FROZEN_MANIFEST)
         endif()
         add_custom_command(
             OUTPUT ${MICROPY_MPYCROSS_DEPENDENCY}
-            COMMAND ${MICROPY_MAKE_EXECUTABLE} -C ${MICROPY_DIR}/mpy-cross
+            COMMAND ${MICROPY_MAKE_EXECUTABLE} -C ${MICROPY_DIR}/mpy-cross USER_C_MODULES=
         )
     endif()
 

py/mkrules.mk

@@ -176,7 +176,7 @@ $(HEADER_BUILD):
 ifneq ($(MICROPY_MPYCROSS_DEPENDENCY),)
 # to automatically build mpy-cross, if needed
 $(MICROPY_MPYCROSS_DEPENDENCY):
-	$(MAKE) -C "$(abspath $(dir $@)..)"
+	$(MAKE) -C "$(abspath $(dir $@)..)" USER_C_MODULES=
 endif
 
 ifneq ($(FROZEN_DIR),)