gindent: Fix doc build to respect docdir

Based on a patch by Elias Pipping:
http://lists.gnu.org/archive/html/bug-indent/2012-01/msg00000.html

Signed-off-by: Lawrence Velázquez <larryv@macports.org>

Index: doc/Makefile.in
===================================================================
--- doc/Makefile.in.orig
+++ doc/Makefile.in
@@ -181,7 +181,7 @@ build_vendor = @build_vendor@
 builddir = @builddir@
 datadir = @datadir@
 datarootdir = @datarootdir@
-docdir = $(prefix)/doc/@PACKAGE@
+docdir = @docdir@
 dvidir = $(docdir)
 exec_prefix = @exec_prefix@
 host = @host@