build: avoid journald on rhel 5

build: avoid journald on rhel 5

Commit f6430390 broke builds on RHEL 5, where glibc (2.5) is too
old to support mkostemp (2.7) or htole64 (2.9).  While gnulib
has mkostemp, it still lacks htole64; and it's not worth dragging
in replacements on systems where journald is unlikely to exist
in the first place, so we just use an extra configure-time check
as our witness of whether to attempt compiling the code.

* src/util/logging.c (virLogParseOutputs): Don't attempt to
compile journald on older glibc.
* configure.ac (AC_CHECK_DECLS): Check for htole64.