patch 8.2.1792: Configure does not recognize Racket 6.1+

Problem:    Configure does not recognize Racket 6.1+.
Solution:   Add a check for "rktio". (closes #7062)
diff --git a/src/configure.ac b/src/configure.ac
index d262990..457f2b8 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -880,6 +880,9 @@
 	  MZSCHEME_CFLAGS="-DMZ_PRECISE_GC"
 	elif test -f "${path}/libracket3m.a"; then
 	  MZSCHEME_LIBS="${path}/libracket3m.a"
+	  if test -f "${path}/librktio.a"; then
+	    MZSCHEME_LIBS="${MZSCHEME_LIBS} ${path}/librktio.a"
+	  fi
 	  MZSCHEME_CFLAGS="-DMZ_PRECISE_GC"
 	elif test -f "${path}/libracket.a"; then
 	  MZSCHEME_LIBS="${path}/libracket.a ${path}/libmzgc.a"