Recent addition of the GCC __attribute__(alloc_size) to some functions
in mem.h broke the build when using Clang 3.5. This commit introduces
configure-time checks for all GCC attributes used in the Serval source
code, and adapts accordingly.
If no instance directory specified, then use paths like
/etc/serval/serval.conf
/var/cache/serval
/var/log/serval
/var/run/serval
etc. for files, instead of all in a single directory.
Log all directory creation as INFO messages.
Interpretation of log.file.directory_path has changed slightly.
Updated servald configuration tech doc.
