Determining if the include file execinfo.h exists failed with the following output:
Change Dir: /var/tmp/portage/net-fs/cvmfs-2.9.0/work/cvmfs-2.9.0_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/ninja cmTC_57437 && [1/2] Building C object CMakeFiles/cmTC_57437.dir/CheckIncludeFile.c.o
FAILED: CMakeFiles/cmTC_57437.dir/CheckIncludeFile.c.o 
/usr/bin/x86_64-gentoo-linux-musl-gcc -D__XATTR_H__   -O2 -pipe -march=x86-64 -frecord-gcc-switches -fno-diagnostics-color -fmessage-length=0 -Os -g -fno-strict-aliasing -fasynchronous-unwind-tables -fno-omit-frame-pointer -fwrapv -fvisibility=hidden -Wall  -fno-optimize-sibling-calls -D_REENTRANT -D__EXTENSIONS__ -D_LARGEFILE64_SOURCE -D__LARGE64_FILES -o CMakeFiles/cmTC_57437.dir/CheckIncludeFile.c.o -c /var/tmp/portage/net-fs/cvmfs-2.9.0/work/cvmfs-2.9.0_build/CMakeFiles/CMakeTmp/CheckIncludeFile.c
/var/tmp/portage/net-fs/cvmfs-2.9.0/work/cvmfs-2.9.0_build/CMakeFiles/CMakeTmp/CheckIncludeFile.c:1:10: fatal error: execinfo.h: No such file or directory
    1 | #include <execinfo.h>
      |          ^~~~~~~~~~~~
compilation terminated.
ninja: build stopped: subcommand failed.