Fix bogus %name-prefix option syntax in all our Bison files.

%name-prefix doesn't use an "=" sign according to the Bison docs, but it
silently accepted one anyway, until Bison 3.0.  This was originally a
typo of mine in commit 012abebab1, and we
seem to have slavishly copied the error into all the other grammar files.

Per report from Vik Fearing; analysis by Peter Eisentraut.

Back-patch to all active branches, since somebody might try to build
a back branch with up-to-date tools.

contrib/cube/cubeparse.y

@@ -39,7 +39,7 @@ static NDBOX * write_point_as_box(char *s, int dim);
 /* BISON Declarations */
 %parse-param {NDBOX **result}
 %expect 0
-%name-prefix="cube_yy"
+%name-prefix "cube_yy"
 
 %token CUBEFLOAT O_PAREN C_PAREN O_BRACKET C_BRACKET COMMA
 %start box

contrib/seg/segparse.y

@@ -42,7 +42,7 @@
 /* BISON Declarations */
 %parse-param {SEG *result}
 %expect 0
-%name-prefix="seg_yy"
+%name-prefix "seg_yy"
 
 %union {
   struct BND {