This adds a new library fishlib, which the CMake build builds.
This library is linked by the tests, fish, and fish_indent, so
that object files do not have to be built separately for each
of them.
This adds support for creating the FISH-BUILD-VERSION-FILE in the CMake
build. A FISH-BUILD-VERSION-FILE is created in the CMake directory
and only updated when necessary.
This adds a new script build_tools/build_lexicon_filter.sh
that builds the lexicon filter. It is factored out from the Makefile,
and both the Makefile and CMake build invoke it.