configure: break up AC_CHECK_HEADERS into one header-file per line

Break up AC_CHECK_HEADERS macro into one header-file per line so it's
easier to read and make changes.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
common: introduce get_linux_version()

get_linux_version() returns a version of the currently running kernel,
encoded as in int, and is contained in common/linux_version.[ch].

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
FileJournal: switch to get_linux_version()

For the purposes of FileJournal::_check_disk_write_cache(), use
get_linux_version(), which is based on uname(2), instead of parsing the
contents of /proc/version.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
FileJournal: use pclose() to close a popen() stream

In FileJournal::_check_disk_write_cache(), use pclose() instead of
fclose() to close a stream, created by popen().

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #951 from ceph/wip-linux-version

common: introduce get_linux_version()

Reviewed-by: Sage Weil <sage@inktank.com>