Fwd: Ambiguity when compiling C code

Could you help me diagnose this error:

In my Jamfile I have:

obj auto_diff : auto_diff.c
              : <include>.
              ;

When I do

b2 auto_diff

I get

Error: ambiguity found when searching for best transformation
Trying to produce type 'OBJ' from:
 -  { auto_diff.c.C }
Generators that succeeded:
 -  darwin.compile.c++
 -  darwin.compile.c
First generator produced:
 -  { darwin%darwin.compile.c++-auto_diff.o.OBJ {
swig%swig.generate-code-auto_diff.cpp.CPP {
darwin%darwin.compile.c.preprocess-auto_diff.i.PREPROCESSED_C {
auto_diff.c.C } } } }

Second generator produced:
 -  { darwin%darwin.compile.c-auto_diff.o.OBJ { auto_diff.c.C } }

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