Merge branch 'ps/clar-unit-test'

Merge branch 'ps/clar-unit-test'

Import clar unit tests framework libgit2 folks invented for our
use.

* ps/clar-unit-test:
  Makefile: rename clar-related variables to avoid confusion
  clar: add CMake support
  t/unit-tests: convert ctype tests to use clar
  t/unit-tests: convert strvec tests to use clar
  t/unit-tests: implement test driver
  Makefile: wire up the clar unit testing framework
  Makefile: do not use sparse on third-party sources
  Makefile: make hdr-check depend on generated headers
  Makefile: fix sparse dependency on GENERATED_H
  clar: stop including `shellapi.h` unnecessarily
  clar(win32): avoid compile error due to unused `fs_copy()`
  clar: avoid compile error with mingw-w64
  t/clar: fix compatibility with NonStop
  t: import the clar unit testing framework
  t: do not pass GIT_TEST_OPTS to unit tests with prove