build: fix cygwin strerror_r failure

build: fix cygwin strerror_r failure

Building the 0.8.8 release candidate on cygwin produced this compiler
warning, which is indicative of catastrophic failure on any attempt to
print an error message with errno turned to a string:

  CC       strerror_r.lo
strerror_r.c: In function 'rpl_strerror_r':
strerror_r.c:67: warning: assignment makes integer from pointer without a cast

This has been fixed in gnulib.

* .gnulib: Update to latest, for strerror_r fix.
* src/util/memory.c (includes): Satisfy 'make syntax-check'.