crypto/compat/issetugid_linux.c: make compatible with uClibc and musl

uClibc does not provide getauxval() and does not have gnu/libc-version.h.
musl does provide getauxval() but also doesn't have gnu/libc-version.h.
This patch incorporates this into issetugid_linux.c so that libressl
builds on glibc, uClilbc and musl.