Apply the following changes:
* Set CI_CMAKE_GENERATOR to "Ninja" by default in ci_cmake.sh.
* Set CI_CC to cc by default in ci_autotools.sh.
* Rename CI_SYSNAME to CI_SYSTEM_NAME; add CI_MACHINE_NAME.
* Apply other minor changes and fixes.
Implement the following updates:
* Add CI_SYSNAME and set it to `uname -s`.
* Set the default CI_CC value to "clang" on all BSD systems.
* Replace CI_SRCDIR_REL_BUILDDIR and CI_INSTALLDIR_REL_BUILDDIR
with CI_SRCDIR_NATIVE and CI_INSTALLDIR_NATIVE.
The former variables were hard-coded and error-prone; the new
variables are computed and safe to use.
The Unix paths in Bash do not mix well with the Windows paths in CMake.
The uppercase and the lowercase environment variables do not mix well
in MSBuild. Et cetera.
On AppVeyor CI, verification of Windows builds with the Visual Studio
toolchain used to work, perhaps due to one or more lucky coincidences,
but it stopped working after an upgrade on the AppVeyor CI site.
Update ci_cmake.sh as follows:
* Use POSIX commands like "mkdir" and "rm" instead of CMake commands
like "make_directory" and "remove_directory".
* Avoid using absolute paths in the CMake command line; use relative
paths that are accessible from both Bash and Windows.
* Clean up incidental mixtures of Windows and Bash-on-Windows
environment variables like {$TEMP,$Temp,$temp} and {$TMP,$Tmp,$tmp}.
For ci_autotools.sh, customize CPP, CPPFLAGS, AR, RANLIB, LD, LDFLAGS
via CI_CPP, CI_CPP_FLAGS, CI_AR, CI_RANLIB, CI_LD, CI_LD_FLAGS.
For ci_cmake.sh and ci_legacy.sh, customize AR, AR_RC and RANLIB via
CI_AR and CI_RANLIB.
Rewrite portions of ci_legacy.sh to match the style of ci_cmake.sh.
Update the ci_*.sh scripts and move them to the more conventional
ci/ directory. Update .appveyor.yml and .travis.yml, as well as
the AUTHORS file, accordingly.
Speed up ci_cmake.sh: add "-DPNG_TESTS=OFF" to the list of CMake
variables if CI_NO_TEST is true.
Remove "sudo: false" from .travis.yml.
Refactor .appveyor.yml.
