The ucontext functionality is not available on all CPUs with all C
libraries. Instead of making just assumptions based on the CPU
architecture, this commit adds the necessary checks in wscript to verify
the availability of the ucontext functionality, before using it in
dbus/sigsegv.c.

This avoids the long list of architecture exclusions, and make it more
robust when building jack2 for new CPU architectures.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>