this removes the following configure options:
 * ZIG_LIBC_LIB_DIR
 * ZIG_LIBC_STATIC_LIB_DIR
 * ZIG_LIBC_INCLUDE_DIR
 * ZIG_DYNAMIC_LINKER
 * ZIG_EACH_LIB_RPATH
 * zig's reliance on CMAKE_INSTALL_PREFIX

these options are still available as command line options, however,
the default will attempt to execute the system's C compiler to
collect system defaults for these values.

closes #870