• The build flags for the C part of the library overwrite the compile flags for C++, in particular, they add -std=c99 to the C++ compilation.
• The C parts get auto-mocced (!) which generates a C++ file (although it's empty), which then gets compiled with the C flags (and weird -std= setting).
• For clang, that leads to a build failure.

So switch out some things:

• Build the C parts as an OBJECT library,
• Don't apply the C flags to anything but that OBJECT library,
• and don't automoc C code.

It now builds when CMake 3.15.2 is used.