build-sys:

build-sys:

While the previous commit checked for a GNU environment so Hurd
compilied ok, this tripped up Cygwin. configure now explicitly tests
for the structure field rather than trying to guess through compilier
flags about what the environment tells us about signals.h

References:
 commit d39d9db07974f4f09cd2f31137742a7ea3be65c5

Signed-off-by: Craig Small <csmall@dropbear.xyz>