use proper version of install in doc/Makefile

Need to do autoconf correctly, bah on Lennart.
diff --git a/doc/Makefile.in b/doc/Makefile.in
index f4b04b8..23bfb06 100644
--- a/doc/Makefile.in
+++ b/doc/Makefile.in
@@ -2,7 +2,7 @@
 DESTDIR=
 KERNEL_HEADERS=-I@KERNEL_HEADERS@
 
-INSTALL=install -s
+INSTALL=@INSTALL@
 
 prefix=@prefix@
 exec_prefix=@exec_prefix@