meson: Search for execinfo.h

meson: Search for execinfo.h

Rather than checking __GLIBC__/__UCLIBC__ macros as a proxy for
execinfo.h presence, just check directly. This allows the build to work
on musl.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>