There's no way to reference C defines from inside a .api file. This is on purpose -- eventually we hope to be able to use other languages (like Java) to access Legato APIs as well.
However you can create an API file from your C header. E.g. something like:
gcc -E -dD some_header.h |sed -r s/[ \t]*#define[ \t]+([A-Za-z][A-Za-z0-9_]*)[ \t]+([0-9]+)[ \t]*$/DEFINE \1 = \2;/;t;d' > some_header.api
should work (assuming you only want #defines which are integers, excluding defines which start with '_'). Of course, you will need to rerun this every time your header changes.