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.
Fix autodetection and use of the cygpath executable on Windows.
Add a new variable CI_CMAKE_TOOLCHAIN_FILE to ci_verify_cmake.sh.
This should help in future cross-platform testing.
Remove the implicit initialization of CI_CC, CI_LD and CI_LIBS from
ci_verify_makefiles.sh. This should help the testing of default values
of their respective make variables (CC, LD and LIBS).
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.
