snapshot-20190921

Do not treat cd "" as equivalent to cd ".".

snapshot-20190627

Various portability fixes.

Compiler portability:

- Do not use gcc's -include option.
  * Include "config.h" in each source file.
  * Update generators to emit #include "config.h" too.
- Do not use gcc's -x option.
  * Use $(CPP) - instead, reading input from stdin.
- Do not hard-code gcc's -Wall option.
  * Use autoconf-archive's AX_CFLAGS_WARN_ALL instead.
- Do not include unused host headers in build utilities.

Standard library compatibility:

- Do not include traditional but non-standard headers <sys/ioctl.h>,
  <sys/param.h>.
  * Change MAXPATHLEN to PATH_MAX.
- Check header files for missing #includes that may be implicitly included
  by other headers.
  * As this causes some header files to be included multiple times, add
    missing header guards to all files.

Make compatibility:

- Do not use $(patsubst), $< and $^, and static pattern rules in Makefiles.