Rework logic to determine whether to use bundled or system libcapnp.
Rework logic to determine whether to use bundled or system libcapnp.

The current logic unconditionally tries to build /and run/ a small
executable linked against libcapnp. The 'run' step is ostensibly
to get the current library version number, however this number isn't
actually used anywhere.

More importantly, this breaks cross-compiling completely as in that
mode you can't run an executable that's just been built.

One option would be to fix the build scripts for cross-compiling but
this is relatively complex. This patch implements a simple change
that basically boils down to '--force-system-libcapnp' actually
forcing that, skipping the build and run tests. The assumption is
that if you're cross-compiling then you're most likely to be building
the library yourself anyway and if you're specifying that flag, then
you know what you're doing.

SymbiFlow/pycapnp,jparyani/pycapnp,SymbiFlow/pycapnp,jparyani/pycapnp,jparyani/pycapnp,SymbiFlow/pycapnp,SymbiFlow/pycapnp,jparyani/pycapnp