Makefile: Provide PACKAGE_TARNAME for autoconf default docdir

docdir was previously being set to "${prefix}/share/doc/${PACKAGE_NAME}"
when it wasn't explicitely set on configuration using --docdir. Without
this appearing in the Makefile, some files silently get silently
installed directly into ${prefix}/share/doc instead within a fish
subdirectory.

I also added datarootdir to fix an autoconf warning,  since autoconf
normally would use it for the directory paths (e.g. docdir =
${datarootdir}/doc/${PACKAGE_TARNAME}). The autoconf generated configure
script has a hack to fix this, but states: "FIXME: This hack should be
removed a few years after 2.60."

Makefile.in

@@ -31,6 +31,8 @@
 # hallucinations.
 #
 
+# Used by docdir
+PACKAGE_TARNAME = @PACKAGE_TARNAME@
 
 #
 # Programs
@@ -46,6 +48,7 @@ INSTALL:=@INSTALL@
 
 prefix = @prefix@
 exec_prefix = @exec_prefix@
+datarootdir = @datarootdir@
 datadir = @datadir@
 bindir = @bindir@
 mandir = @mandir@