18.104.22.168: fix build on OpenBSD/i386 -current and upcoming 4.8 release
* Patch by Josh Elsasser, lp#615489.
* On OpenBSD/i386, the structure that stores the FPU state during
signals was recently moved.
Add a script to tools-for-build/ which is run from make-config.sh
to detect where the FPU state is located, and create a
src/runtime/openbsd-sigcontext.h header containing a #define which
src/runtime/x86-bsd-os.c can use.
This is necessary to build SBCL on recent -current snapshots of
OpenBSD, and for the upcoming 4.8 release.