add ifdefs to support building the extension loader with older headers

[bashbaug]

Fixes #7

Description of Changes

Adds ifdefs to the extension loader itself so it will build with older headers that do not include all extensions in the XML file.

When older headers are used a message will be emitted indicating that a define for an extension was not found, but the build will continue.

Testing Done

Commented out header files and verified that the extension loader will still build.

[bashbaug]

The message in the CI builds for the POCL extension is expected because this extension is not in the upstream headers (yet).