handle deficient signal systems

Pull in the sigaction module from gnulib, and add ifdef checks
for a bunch of signals that don't exist on Windows.
diff --git a/autogen.sh b/autogen.sh
index de06993..2c75d4b 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -13,6 +13,7 @@
 	iswblank
 	lstat
 	regex
+	sigaction
 	snprintf-posix
 	stdarg
 	strcase