libnspr: fix build with the musl C library

Even though libnspr uses a configure script, it does look at __GLIBC__
and other macros to determine whether certain C library features are
available or not. Such things fails completely with the musl C
library, since it doesn't define __GLIBC__.

To fix this, we borrow some logic from Alpine Linux, which consists in
explicitly specifying which features are available in the musl C
library.

Fixes:

  http://autobuild.buildroot.org/results/ddd/dddd032232126f1e476e6aea2c6a32e9cd1ddfae/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>