Fixes build against uClibc.

uClibc provides 2 versions of clock_gettime(), one with realtime
support and one without (this is so you can avoid linking in -lrt
unless actually needed). This means that the clock_gettime() don't
need -lrt. We still need it for timer_create() so we check for this
function in addition.

We also need check if -lm is needed for isnan().

Both -lm and -lrt are needed for libs_qga.
Signed-off-by: NNatanael Copa <ncopa@alpinelinux.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>