Use __UCLIBC__ when checking for GLIBC features

Commit f0f85d8d introduces __GLIBC__ to check for glibc only features.
However this is not sufficient for uClibc because it shares code with
glibc.  To select for features in glibc but not uClibc, we need
defined(__GLIBC__) && !defined(__UCLIBC__).

https://bugzilla.gnome.org/show_bug.cgi?id=764021