Fix more bugs in specific tests under AC_LANG(C++).

Found by exhaustive testing for differences between probe results
under AC_LANG(C) and AC_LANG(C++).

 * lib/autoconf/c.m4 (AC_C_FLEXIBLE_ARRAY_MEMBER): Cast result of
   malloc for C++ compatibility.
 * lib/autoconf/programs.m4 (_AC_PROG_LEX_YYTEXT_DECL): Declare yywrap
   as extern "C" when compiling as C++.

lib/autoconf/c.m4

@@ -1993,8 +1993,8 @@ AC_DEFUN([AC_C_FLEXIBLE_ARRAY_MEMBER],
 	    #include <stddef.h>
 	    struct s { int n; double d[]; };]],
 	  [[int m = getchar ();
-	    struct s *p = malloc (offsetof (struct s, d)
-				  + m * sizeof (double));
+	    struct s *p = (struct s *) malloc (offsetof (struct s, d)
+					       + m * sizeof (double));
 	    p->d[0] = 0.0;
 	    return p->d != (double *) NULL;]])],
        [ac_cv_c_flexmember=yes],

lib/autoconf/programs.m4

@@ -747,6 +747,9 @@ fi])])
 AC_DEFUN([_AC_PROG_LEX_YYTEXT_DECL],
 [cat >conftest.l <<_ACEOF[
 %{
+#ifdef __cplusplus
+extern "C"
+#endif
 int yywrap(void);
 %}
 %%