ci: Update the CI scripts

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.
3 files changed