zhongpan

add support for gcovr+gcc(include mingw)

2

gcovr is easier to use than lcov and better cross-platform. I implemented gcovr support by referring to the gcov code, and tested gcc, including the MinGW platform.

Avoid gcov filenames too long

1

In PR [add support for Ninja #22](https://github.com/RWTH-HPC/CMake-codecov/pull/22), I realized that I had introduced two issues: 1. The gcov file name may exceed the maximum length limit 2. When build gcov...