yacc: fix relocatability

[Akim Demaille]

commit 586e2ab0c83d146163b22ba6fbdfcaa472e28eed
Author: Akim Demaille <address@hidden>
Date:   Sat Jan 12 10:39:55 2019 +0100

    yacc: fix relocatability
    
    * src/yacc.in (prefix): Define it, as it's typically needed for
    exec_prefix.

diff --git a/src/yacc.in b/src/yacc.in
index 1d45123d..3ee622b3 100644
--- a/src/yacc.in
+++ b/src/yacc.in
@@ -2,6 +2,7 @@
 
 @relocatable_sh@
 if test "@RELOCATABLE@" = yes; then
+  prefix="@prefix@"
   exec_prefix="@exec_prefix@"
   bindir="@bindir@"
   orig_installdir="$bindir" # see Makefile.am's *_SCRIPTS variables
@@ -18,5 +19,7 @@ else
   }
 fi
 
address@hidden@
address@hidden@
 bindir=`relocate "@bindir@"`
 exec "$bindir/bison" -y "$@"