From Makefile.tpl:
EXTRA_TARGET_FLAGS = \
'AR=$$(AR_FOR_TARGET)' \
'AS=$(COMPILER_AS_FOR_TARGET)' \
'CC=$$(CC_FOR_TARGET) $$(XGCC_FLAGS_FOR_TARGET) $$(TFLAGS)' \
'CFLAGS=$$(CFLAGS_FOR_TARGET)' \
'CXX=$$(CXX_FOR_TARGET) $$(XGCC_FLAGS_FOR_TARGET) $$(TFLAGS)' \
'CXXFLAGS=$$(CXXFLAGS_FOR_TARGET)' \
'DLLTOOL=$$(DLLTOOL_FOR_TARGET)' \I think AS=$(...) is wrong.
