Refactor ax_code_coverage.m4
Refactor ax_code_coverage.m4

Adapt the ax_code_coverage.m4 to the project's
build structure

- Formatting fixes and clean up
- Change variable names and documentation so
gcov is not referred as the default coverage
tool
- Use the term cov-tool instead of gcov
- Add maintainer to author list
- Separate code coverage support into compilation,
  capture and report generation

sociomantic/libdrizzle-redux,sociomantic/libdrizzle-redux,sociomantic/libdrizzle-redux