irmp: workaround incomplete platform detection when under libtool

The upstream IRMP project builds fine with direct gcc(1) invocation.
While the same imported source fails detection and then defaults to AVR
when built under libtool in the libsigrokdecode setup.

Provide the symbols which IRMP logic expects, to reduce changes against
upstream sources. Derive these symbols from conditions that are checked
in the sigrok project in other locations, too.