fix: do not emit nested comments

With input such as

    %token<fl> yVL_CLOCK "/*verilator sc_clock*/"

we generate

    yVL_CLOCK = 610,      /* "/*verilator sc_clock*/"  */

which is invalid since the comment will actually be closed on the
first "*/".  Let's turn "*/" into "*\/" to avoid this.  But GCC will
also warn about "/*" inside a comment, so let's "escape" it too.

Reported by Huang Rui.
https://github.com/akimd/bison/issues/38

* data/skeletons/c-like.m4 (_b4_comment): Escape comment delimiters in
comments.
* tests/input.at (Torturing the Scanner): Check thes cases.
* tests/m4.at: New.

data/skeletons/c-like.m4

@@ -22,10 +22,14 @@
 # Put TEXT in comment.  Avoid trailing spaces: don't indent empty lines.
 # Avoid adding indentation to the first line, as the indentation comes
 # from OPEN.  That's why we don't patsubst([$1], [^\(.\)], [   \1]).
+# Turn "*/" in TEXT into "* /" so that we don't unexpectedly close
+# the comments before its end.
 #
 # Prefix all the output lines with PREFIX.
 m4_define([_b4_comment],
-[$2[]m4_bpatsubst(m4_expand([[$1]]), [
+[$2[]m4_bpatsubsts(m4_expand([$1]),
+                   [[*]/], [*\\/],
+                   [/[*]], [/\\*], [
 \(.\)], [
 $3\1])$4])