build: drop the ldexp gnulib module

build: drop the ldexp gnulib module

The ldexp gnulib module adds "-lm" to the $LIBS variable if-and-only-if
the ldexp() function require linking to libm. There is no harm in
linking to libm even if it isn't required for ldexp(), so simply drop
the gnulib module.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>