XL,Linux: cmake 3.10.1 misidentifies IBM XL 13.1.7 as clang 4.0.1

When configuring a cmake project with cmake 3.10.1 using the XL C and C++ compilers, cmake identifies the compiler as clang/clang++:

$ CC=xlc cmake ..
-- The C compiler identification is Clang 4.0.1
[...]
-- Check for working C compiler: /usr/bin/xlc
-- Check for working C compiler: /usr/bin/xlc -- works
[...]
$ xlc --version
IBM XL C/C++ for Linux, V13.1.7 (Beta 2)
Version: 13.01.0007.0000

Due to this misidentification, cmake is also not able to find the correct options to enable OpenMP etc.