Resolve "Make Clang the default compiler in Makefile and CMakeLists.txt"
- Related issues: #1537
- Requested reviewers: @multrus @sagnowski @pihlajakuja
Reason why this change is needed
- As clang 18 is the reference compiler now, it should be the default in our build systems
Description of the change
- change default compiler in Makefile and CMakeLists.txt to clang
- add option for forcing gcc
- keep gcc when building with GCOV
Affected operating points
- None, build system only change
Related to #1537