Oops, exporting CXX for AC_CONFIG_SUBDIRS doesn't work if the user specified their own CXX, in which case their version will be in ac_configure_args and will take precedence.  We'll have to modify ac_configure_args.

Install schema.capnp and c++.capnp to $PREFIX/include/capnp.  Make sure /usr/include and /usr/local/include are in the capnpc search path.  Fix Makefile bug related to ridiculous gmake implicit rules.  And update the docs a bit.

Disable use of unit analysis (via kj::Quantity) by default (since it has some overhead), but enable it specifically for dev builds.