Fix conflicting declaration on Solaris

SunC
```
"cont.c", line 24: identifier redeclared: madvise
	current : function(pointer to char, unsigned int, int) returning int
	previous: function(pointer to void, unsigned int, int) returning int : "/usr/include/sys/mman.h", line 232
```

GCC
```
cont.c:24:12: error: conflicting types for 'madvise'
   24 | extern int madvise(caddr_t, size_t, int);
      |            ^~~~~~~
In file included from cont.c:16:
/usr/include/sys/mman.h:232:12: note: previous declaration of 'madvise' was here
  232 | extern int madvise(void *, size_t, int);
      |            ^~~~~~~
```

configure.ac

@@ -1108,6 +1108,15 @@ main()
 		  ])
 		])
 		AC_CHECK_TYPES([caddr_t],[],[],[@%:@include <sys/types.h>])
+		RUBY_WERROR_FLAG([AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+		    @%:@if defined(HAVE_CADDR_T) && (defined(MADV_FREE) || defined(MADV_DONTNEED))
+		    @%:@include <sys/unistd.h>
+		    @%:@include <sys/mman.h>
+		    @%:@include <sys/types.h>
+		    extern int madvise(caddr_t, size_t, int);
+		    ]], [[]])],
+		    [AC_DEFINE(NEED_MADVICE_PROTOTYPE_USING_CADDR_T, 1)], [])
+		])
 		],
 [haiku*], [
 		LIBS="$LIBS" # m lib is include in root

cont.c

@@ -18,8 +18,7 @@
 
 // On Solaris, madvise() is NOT declared for SUS (XPG4v2) or later,
 // but MADV_* macros are defined when __EXTENSIONS__ is defined.
-#if defined(__sun) && defined(HAVE_CADDR_T) && \
-    (defined(MADV_FREE) || defined(MADV_DONTNEED))
+#ifdef NEED_MADVICE_PROTOTYPE_USING_CADDR_T
 #include <sys/types.h>
 extern int madvise(caddr_t, size_t, int);
 #endif