gnulib: Copy in error(3) replacement from gnulib

This is available under a compatible license (LGPLv2+).

m4/guestfs-libraries.m4

@@ -41,13 +41,14 @@ dnl Headers.
 AC_CHECK_HEADERS([\
     byteswap.h \
     endian.h \
-    sys/endian.h \
+    error.h \
     errno.h \
     linux/fs.h \
     linux/magic.h \
     linux/raid/md_u.h \
     linux/rtc.h \
     printf.h \
+    sys/endian.h \
     sys/inotify.h \
     sys/mount.h \
     sys/resource.h \
@@ -66,6 +67,7 @@ dnl Functions.
 AC_CHECK_FUNCS([\
     accept4 \
     be32toh \
+    error \
     fsync \
     futimens \
     getprogname \