The removal of exec_prefix in a900f200
caused the generated .pc files to be unusable when built with autotools.

The generated libdir value would read:

  libdir=${exec_prefix}/lib

but since exec_prefix was unspecified, pkg-config would choke:

  zarvox@localhost $ pkg-config capnp-rpc --cflags --libs
  Variable 'exec_prefix' not defined in '/usr/local/lib/pkgconfig/capnp-rpc.pc'