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