Provide a fallback implementation of memrchr

Move the local fallback out of string.c so other source files can use
memrchr on platforms that do not provide it.

configure.ac

@@ -2131,6 +2131,7 @@ AC_REPLACE_FUNCS(flock)
 AC_REPLACE_FUNCS(hypot)
 AC_REPLACE_FUNCS(lgamma_r)
 AC_REPLACE_FUNCS(memmove)
+AC_REPLACE_FUNCS(memrchr)
 AC_REPLACE_FUNCS(nan)
 AC_REPLACE_FUNCS(nextafter)
 AC_REPLACE_FUNCS(setproctitle)
@@ -2246,7 +2247,6 @@ AC_CHECK_FUNCS(mblen)
 AC_CHECK_FUNCS(memalign)
 AC_CHECK_FUNCS(memset_s)
 AC_CHECK_FUNCS(writev)
-AC_CHECK_FUNCS(memrchr)
 AC_CHECK_FUNCS(memmem)
 AC_CHECK_FUNCS(mkfifo)
 AC_CHECK_FUNCS(mknod)

include/ruby/missing.h

@@ -183,6 +183,10 @@ RUBY_EXTERN int memcmp(const void *, const void *, size_t);
 RUBY_EXTERN void *memmove(void *, const void *, size_t);
 #endif
 
+#ifndef HAVE_MEMRCHR
+RUBY_EXTERN void *memrchr(const void *, int, size_t);
+#endif
+
 /*
 #ifndef HAVE_MODF
 RUBY_EXTERN double modf(double, double *);

missing/memrchr.c

@@ -0,0 +1,12 @@
+#include "ruby/missing.h"
+
+void *
+memrchr(const void *ptr, int ch, size_t len)
+{
+    const unsigned char *p = (const unsigned char *)ptr + len;
+
+    while (p > (const unsigned char *)ptr) {
+        if (*--p == (unsigned char)ch) return (void *)p;
+    }
+    return NULL;
+}

string.c

@@ -4763,19 +4763,6 @@ rb_str_byteindex_m(int argc, VALUE *argv, VALUE str)
     return Qnil;
 }
 
-#ifndef HAVE_MEMRCHR
-static void*
-memrchr(const char *search_str, int chr, long search_len)
-{
-    const char *ptr = search_str + search_len;
-    while (ptr > search_str) {
-        if ((unsigned char)*(--ptr) == chr) return (void *)ptr;
-    }
-
-    return ((void *)0);
-}
-#endif
-
 static long
 str_rindex(VALUE str, VALUE sub, const char *s, rb_encoding *enc)
 {

win32/Makefile.sub

@@ -320,7 +320,7 @@ LIBS = $(LIBS) imagehlp.lib shlwapi.lib bcrypt.lib $(EXTLIBS)
 !endif
 !if !defined(MISSING)
 MISSING = crypt.obj ffs.obj langinfo.obj lgamma_r.obj strlcat.obj strlcpy.obj win32/win32.obj win32/file.obj setproctitle.obj
-MISSING = $(MISSING) explicit_bzero.obj
+MISSING = $(MISSING) explicit_bzero.obj memrchr.obj
 !endif
 DLNOBJ = dln.obj