[PATCH] sim: make sure to include strsignal prototype

[PATCH] sim: make sure to include strsignal prototype

Before POSIX standardized strsignal(), old systems would hide the
prototype unless the normal extension defines were enabled.  So use
the AC_USE_SYSTEM_EXTENSIONS helper for that.

Then make sure we include string.h ourselves in nrun.c rather than
relying on implicit includes via other sim headers.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>