configure: properly check if -lrt and -lm is needed

configure: properly check if -lrt and -lm is needed

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: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
(cherry picked from commit 8bacde8d86a09699207d85d4bab06162aed18dc4)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>