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, and changes
the default location from /tmp to /var/cache/asterisk.

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='${sbindir}'
+astcachedir='${localstatedir}/cache/asterisk'
 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=/var/cache/asterisk
         astetcdir=/var/etc/asterisk
         astsbindir=/opt/asterisk/sbin
         astlibdir=/opt/asterisk/lib