ruby: configure mis-detects isnan/isinf on musl

ruby: configure mis-detects isnan/isinf on musl

The configure script does not detect isnan/isinf as macros
as is the case in musl:
    checking for isinf... no
    checking for isnan... no

Backport an upstream patch from 2.7.0-preview1 to address this:
    checking whether isinf is declared... yes
    checking whether isnan is declared... yes

Signed-off-by: André Draszik <andre.draszik@jci.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>