- Followup to last commit:
      - Don't parallelise the perl build as libdummy.so must be built before
        building {cpan,ext}/.
      - Iteratively append ${PKG_CFLAGS_CONFIGURE} to ccflags when building
        perl as environment variables aren't considered by the configure script.
    
    - Add missing build step `build_dir' to build.usage.
    - Force the perk prefix to /usr rather than the default value of /usr/local.