When compiled with gcc the SingletonHolder instance symbols
are marked as globally unique (a GNU extension) but when compiled
with clang the symbols are marked as weak. Python extensions
are loaded by default without either RTLD_GLOBAL or RTLD_LOCAL
specified and the singleton symbols then remain private to each
module leading to multiple singleton instances within the same
process. This fix will also cover older versions of LD and is
a cleaner implementation than that removed in
afa941f0.