Define compat version of mkdtemp for systems lacking it

Solaris 9 doesn't have mkdtemp() so we need to emulate it for the
rsync transport implementation.  Since Solaris 9 is lacking this
function we can also reasonably assume it is not available on
Solaris 8 either.  The new Makfile definition NO_MKDTEMP can be
set to enable the git compat version.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

compat/mkdtemp.c

@@ -0,0 +1,8 @@
+#include "../git-compat-util.h"
+
+char *gitmkdtemp(char *template)
+{
+	if (!mktemp(template) || mkdir(template, 0700))
+		return NULL;
+	return template;
+}