Fix compilation on Armel due to coincident variables R2, R3 and R4 in <sys/ucontext.h>. Based on suggestion by Paul Brook in <URL: http://lists.alioth.debian.org/pipermail/debian-science-maintainers/2011-October/009876.html >. Patch by Anton Gladky <gladk@debian.org>. Patch has been used by the Debian edition of FreeCAD since 2011.