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.
