Don't rely on the existence of execinfo

Hi, one more from the Julia BinaryBuilder:

Building CXX object ThirdParty/loguru/vtkloguru/CMakeFiles/loguru.dir/loguru.cpp.o
cd /workspace/srcdir/build/ThirdParty/loguru/vtkloguru && /opt/bin/aarch64-linux-musl-g++ --sysroot=/opt/aarch64-linux-musl/aarch64-linux-musl/sys-root/  -DVTK_IN_VTK -Dloguru_EXPORTS -I/workspace/srcdir/build/ThirdParty/loguru/vtkloguru -I/workspace/srcdir/vtk-496e01f755421cc12dc52d40d8143299af9c6325/ThirdParty/loguru/vtkloguru -isystem /workspace/srcdir/build/ThirdParty/loguru -isystem /workspace/srcdir/vtk-496e01f755421cc12dc52d40d8143299af9c6325/ThirdParty/loguru  -O3 -DNDEBUG -fPIC -fvisibility=hidden -fvisibility-inlines-hidden   -std=c++11 -o CMakeFiles/loguru.dir/loguru.cpp.o -c /workspace/srcdir/vtk-496e01f755421cc12dc52d40d8143299af9c6325/ThirdParty/loguru/vtkloguru/loguru.cpp
/workspace/srcdir/vtk-496e01f755421cc12dc52d40d8143299af9c6325/ThirdParty/loguru/vtkloguru/loguru.cpp:91:11: fatal error: execinfo.h: No such file or directory
   91 |  #include <execinfo.h>  // for backtrace
      |           ^~~~~~~~~~~~
compilation terminated.

This comes out of cross compiling for a system with the musl libc . Googling of "execinfo.h musl" finds a number of other such cases. As far as I understand it's not POSIX, so may be it can be #ifdef'ed out with a cmake check...