Apply the following changes:
* Update CI_BUILDDIR and CI_INSTALLDIR to reflect the new script
names.
* Introduce CI_SRCDIR_FROM_BUILDDIR and CI_INSTALLDIR_FROM_BUILDDIR
to the cmake build, to ensure that the Windows tools can handle
native Windows paths correctly when executed via Bash-on-Windows.
* Stop guessing and using CI_SRCDIR_NATIVE and CI_INSTALLDIR_NATIVE
in the cmake build, since we now have a better solution.
* Show the ninja program version in ninja-based cmake builds.
* Move the cleanup of previous builds into separate functions, and
do it in a cleaner manner. No pun intended.
Give the CI scripts new names that better reflect what they do, and
make room for activities such as linting or dependency installation:
* Rename ci_autotools.sh to ci_verify_configure.sh.
* Rename ci_cmake.sh to ci_verify_cmake.sh.
* Rename ci_legacy.sh to ci_verify_makefiles.sh.
Also rename CI_LEGACY_MAKEFILES to CI_MAKEFILES.
Update the config files for Travis CI and AppVeyor CI accordingly.