need mechanism for lsbcc to include proper headers for uplifted libs

Our current practice for library uplifts is to include both libraries as
separate specs, with older LSB versions referencing older specs, and newer
versions referencing newer specs (and older, as needed).

This works everywhere except with SDK headers. We only ship one set of headers
for all LSB versions. Supplying two sets of headers for version X and version
X+Y requires some mechanism for picking the right header to include during the
build.

The best place to do this is in lsbcc, with it adding -I arguments to the
underlying compiler to pull in the right header.

This has come up in the context of the png uplift, but is general enough to
apply to any uplift where we need to support multiple versions of a library.