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.

tests/local.mk

@@ -56,6 +56,7 @@ TESTSUITE_AT =                                \
   %D%/java.at                                 \
   %D%/javapush.at                             \
   %D%/local.at                                \
+  %D%/m4.at                                   \
   %D%/named-refs.at                           \
   %D%/output.at                               \
   %D%/package.m4                              \