I've been running with this patch for many years, and it's required to ensure
correct operation of USE_PKGSRC_GCC_RUNTIME, as well as allowing libtool to be
compiler-agnostic, at least within the same compiler family. Bad things happen
otherwise, such as multiple libstdc++.so's being pulled in, and it matches the
behaviour of NetBSD and others.
While here simplify some of the logic, removing support for GCC 2.7(!) and
Solaris 2.5 and earlier(!!), making this section of code easier to read.
Ride previous PKGREVISION bump.
9d34c11159