Introduce astcachedir, to be used for temporary bucket files

As described in the issue, /tmp is not a suitable location for a
large amount of cached media files, since most distributions make
/tmp a RAM-based tmpfs mount with limited capacity.

I opted for a location that can be configured separately, as opposed
to using a subdirectory of spooldir, given the different storage
profile (transient files vs files that might stay there indefinitely).

This commit just makes the cache directory configurable, but leaves
it at /tmp by default, to ensure backwards compatibility.

A future commit that only targets master could change the default
location to something more sensible such as /var/tmp/asterisk. At
that point, the cachedir could be created and cleaned up during
uninstall by the Makefile script.

ASTERISK-29143

Change-Id: Ic54e95199405abacd9e509cef5f08fa14c510b5d

configure

@@ -1295,6 +1295,7 @@ BUILD_OS
 BUILD_VENDOR
 BUILD_CPU
 BUILD_PLATFORM
+astcachedir
 astvarrundir
 astlogdir
 astspooldir
@@ -4660,6 +4661,7 @@ $as_echo "$ac_cv_safe_to_define___extensions__" >&6; }
 
 # System default paths
 astsbindir='${sbindir}'
+astcachedir='/tmp'
 astetcdir='${sysconfdir}/asterisk'
 astheaderdir='${includedir}/asterisk'
 astlibdir='${libdir}'
@@ -4728,6 +4730,7 @@ $as_echo "#define _DARWIN_UNLIMITED_SELECT 1" >>confdefs.h
      ;;
      solaris*)
      if test ${prefix} = 'NONE'; then
+        astcachedir=/tmp
         astetcdir=/var/etc/asterisk
         astsbindir=/opt/asterisk/sbin
         astlibdir=/opt/asterisk/lib