Default imports are not imported by default

[LehMaxence]

capnproto sets the default imports location at build time through the CAPNP_INCLUDE_DIR symbol definition. This symbol is set by autotool (and cmake) to the absolute path $PREFIX/include, this hampers the relocatability of the capnp binary.

Current workaround is to re-include the default imports location when needed, e.g.:
capnpc -I"$CONDA_PREFIX/include" <..>

Note:
conda-build replaces the $PREFIX in files by a placeholder so that it gets replace by the correct value at install time, but this doesn't work for the binary as it created string length issues.

This issue is related to the one here: capnproto/capnproto#1062