Fix bug in cyl_bessel_i that hits when v=0.5 and x is small.

Fix return type of signbit to match C99 std.
Update and regenerate docs.
Fixes #2877.

[SVN r51890]

libs/math

@@ -1 +1 @@
-Subproject commit 34641e453416592bcbaaf965ed26fbcf90318cc8
+Subproject commit 9c124ea11c376ce3856a82dad285bea87d930535