Make sure docdir is defined when making doc/

Apparently some autoconf versions need this, while others don't.
This means documentation will be installed into share/doc/tor/.

doc/Makefile.am

@@ -26,6 +26,8 @@ EXTRA_DIST = HACKING                                         \
 
 nodist_man_MANS = $(asciidoc_files:=.1)
 
+docdir = @docdir@
+
 doc_DATA = $(asciidoc_files:=.html)
 
 asciidoc_product = $(nodist_man_MANS) $(doc_DATA)