libgpiod: bump version to 0.2

libgpiod: bump version to 0.2

New release adds a couple new features and improvements, although the
most important one - the test suite - doesn't really matter for
buildroot.

The project has also been relicensed under LGPLv2.1.

We need the m4 directory fixup now since the AC_CONFIG_MACRO_DIRS([m4])
macro was used in configure.ac, but we don't want to have unnecessary
files in the git repo.

New features:
- relicensed under LGPLv2.1
- implemented a unit testing framework together with a comprehensive
  set of test cases
- added a non-closing variant of the gpiochip iterator and foreach
  macro [by Clemens Gruber]
- added gpiod_chip_open_by_label()

Improvements:
- Makefiles & build commands have been reworked [by Thierry Reding]
- documentation updates
- code shrinkage here and there
- coding style fixes
- removed all designated initializers from the header for better standards
  compliance

Bug fixes:
- fix the return value of gpiod_simple_event_loop()
- don't try to process docs if doxygen is not installed

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>