Clang detection does not work if there is no clang (but e.g. clang-9)

[igchor]

ISSUE: Clang detection does not work if there is no clang (but e.g. clang-9)

Environment Information

• libpmemobj-cpp version(s):
• PMDK (libpmemobj) package version(s):
• OS(es) version(s): Ubuntu 20.04
• kernel version(s):
• compiler, libraries, packaging and other related tools version(s):

and possibly:

• TBB version(s):
• ndctl version(s):

Please provide a reproduction of the bug:

CXX=clang++-9 CC=clang-9 cmake ..

How often bug is revealed:

always

Actual behavior:

CLANG variable in Cmake is not set which results in test compilation errors (since we have conditional library linking under clang - see ctest_helpers.cmake:build_test_atomic)

Expected behavior:

Test compilation passes

Details

Additional information about Priority and Help Requested:

Are you willing to submit a pull request with a proposed change? (Yes, No)

Requested priority: (Showstopper, High, Medium, Low)