fixed default coroutine selection for musl/uclibc

https://github.com/ruby/ruby/pull/3567/commits/b570e7de87aaad8c903176d835e8124127f627b3

Signed-off-by: Waldemar Brodkorb <wbx@opennadk.org>

diff -Nur ruby-2.7.1.orig/configure.ac ruby-2.7.1/configure.ac
--- ruby-2.7.1.orig/configure.ac	2020-03-31 12:44:55.000000000 +0200
+++ ruby-2.7.1/configure.ac	2020-10-01 00:03:53.130462286 +0200
@@ -2344,8 +2344,12 @@
             rb_cv_coroutine=copy
         ],
         [*], [
-            rb_cv_coroutine=ucontext
+            AC_CHECK_FUNCS([getcontext swapcontext makecontext],
+               [rb_cv_coroutine=ucontext],
+               [rb_cv_coroutine=copy; break]
+            )
         ]
+
     )
     AC_MSG_RESULT(${rb_cv_coroutine})
 ])
diff -Nur ruby-2.7.1.orig/coroutine/copy/Context.c ruby-2.7.1/coroutine/copy/Context.c
--- ruby-2.7.1.orig/coroutine/copy/Context.c	2020-03-31 12:44:55.000000000 +0200
+++ ruby-2.7.1/coroutine/copy/Context.c	2020-10-01 00:04:59.414670705 +0200
@@ -5,6 +5,8 @@
  *  Copyright, 2019, by Samuel Williams. All rights reserved.
 */
 
+#include <sys/types.h>
+
 #include "Context.h"
 
 // http://gcc.gnu.org/onlinedocs/gcc/Alternate-Keywords.html