Fix multi-line comment regular expression to handle (non) nested comments.

Ken reminded me of a couple cases that I should be testing. These are
the non-nestedness of things that look like nested comments as well as
potentially tricky things like "/*/" and "/*/*/".

The (non) nested comment case was not working in the case of the
comment terminator with multiple '*' characters. We fix this by not
considering a '*' as the "non-slash" to terminate a sequence of '*'
characters within the comment. We also fix the final match of the
terminator to use '+' rather than '*' to require the presence of a
final '*' character in the comment terminator.

glcpp-lex.l

@@ -53,7 +53,7 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 }
 
 	/* Multi-line comments */
-[/][*]([^*]*[*]+[^/])*[^*]*[*]*[/] {
+[/][*]([^*]*[*]+[^*/])*[^*]*[*]+[/] {
 	if (yyextra->space_tokens)
 		return SPACE;
 }

tests/063-comments.c

@@ -13,3 +13,8 @@ and slashes / *** /
 and other stuff.
 ****/
 more code here
+/* Test that /* nested
+   comments */
+are not treated like comments.
+/*/ this is a comment */
+/*/*/