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.
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.
