Merge from gnulib

Merge from gnulib

This incorporates:
2017-08-09 tempname: do not depend on secure_getenv
2017-08-08 extensions: add _OPENBSD_SOURCE
2017-08-06 manywarnings: Add support for C++
2017-08-06 warnings, manywarnings: Add support for multiple languages
* admin/merge-gnulib: Don't use m4/manywarnings-c++.m4.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lib/secure_getenv.c, m4/secure_getenv.m4: Remove.
* lib/tempname.c, m4/extensions.m4, m4/manywarnings.m4, m4/warnings.m4:
Copy from gnulib.