Update to rra-c-util 7.0 and C TAP Harness 4.2

Update to rra-c-util 7.0 and C TAP Harness 4.2

Update to rra-c-util 7.0:

* Support a warning build under Clang.
* Avoid zero-length allocations in reallocarray and vector.

Update to C TAP Harness 4.2:

* Avoid zero-length allocations in breallocarray.
* Add is_blob and is_bool functions.
* Use C_TAP_SOURCE and C_TAP_BUILD environment variables in tests.

Compile cleanly under GCC 7 and Clang warnings and Clang's static
analyzer.