Mark strlcat() and strlcpy() as weak functions, for libsyslog

The strlcat() and strlcpy() functions are only intended to be used
by syslog.c internally (and syslogd), when building libsyslog.

A user linking with libsyslog may have another library that provides
strlcat() or strlcpy() replacements.  We must therefore mark ours as
weak functions so they can be overridden.

This patch also add a convenience library for libsyslog, to control
the build deps. for libsyslog.  This is where external dependencies
should be addded (explicitly) when syslog.c is updated from NetBSD.
If you add new deps you likely want to mark them too as weak refs.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

lib/strlcat.c

@@ -16,6 +16,10 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#include <config.h>
+#include <compat.h>
+#ifndef HAVE_STRLCAT
+
 #include <sys/types.h>
 #include <string.h>
 
@@ -27,7 +31,7 @@
  * If retval >= dsize, truncation occurred.
  */
 size_t
-strlcat(char *dst, const char *src, size_t dsize)
+__strlcat(char *dst, const char *src, size_t dsize)
 {
 	const char *odst = dst;
 	const char *osrc = src;
@@ -53,3 +57,7 @@ strlcat(char *dst, const char *src, size_t dsize)
 
 	return(dlen + (src - osrc));	/* count does not include NUL */
 }
+
+weak_alias(__strlcat, strlcat);
+
+#endif /* HAVE_STRLCAT */