forestdb builds break on newer linux kernels

In recent glibc versions the struct ucontext was renamed to ucontext_t. The details can be found at [1]. The change is compatible with older glibc versions because the typedef for the struct has been called ucontext_t for a long while. So it will be picked up instead of the struct tag. [1] https://sourceware.org/bugzilla/show_bug.cgi?id=21457