Can't recursively search DLLs with FFI on Cygwin

* doc/ref/api-foreign.text (dynamic-link): document problems with recursive DLLs.
* test-suite/standalone/test-ffi (global): with Cygwin, dynamic-link C library explicitly
* test-suite/standalone/test-foreign-object-scm (libc-ptr): with Cygwin, link C library explicitly
* test-suite/tests/foreign.test (qsort): with Cygwin, link C library explicitly

test-suite/standalone/test-ffi

@@ -3,7 +3,7 @@ exec guile -q -s "$0" "$@"
 !#
 ;;; test-ffi --- Foreign function interface.         -*- Scheme -*-
 ;;;
-;;; Copyright (C) 2010 Free Software Foundation, Inc.
+;;; Copyright (C) 2010, 2017 Free Software Foundation, Inc.
 ;;;
 ;;; This library is free software; you can redistribute it and/or
 ;;; modify it under the terms of the GNU Lesser General Public
@@ -263,7 +263,15 @@ exec guile -q -s "$0" "$@"
 (if (defined? 'setlocale)
     (setlocale LC_ALL "C"))
 
-(define global (dynamic-link))
+(define global (cond
+                ((string-contains %host-type "cygwin")
+                 ;; On Cygwin, dynamic-link doesn't search recursively
+                 ;; into linked DLLs. Thus one needs to link to the core
+                 ;; C library DLL explicitly.
+                 (dynamic-link "cygwin1"))
+                (else
+                 (dynamic-link))))
+
 
 (define strerror
   (pointer->procedure '* (dynamic-func "strerror" global)

test-suite/standalone/test-foreign-object-scm

@@ -3,7 +3,7 @@ exec guile -q -s "$0" "$@"
 !#
 ;;; test-foreign-object-scm --- Foreign object interface.     -*- Scheme -*-
 ;;;
-;;; Copyright (C) 2014 Free Software Foundation, Inc.
+;;; Copyright (C) 2014, 2017 Free Software Foundation, Inc.
 ;;;
 ;;; This library is free software; you can redistribute it and/or
 ;;; modify it under the terms of the GNU Lesser General Public
@@ -26,7 +26,17 @@ exec guile -q -s "$0" "$@"
 
 (define (libc-ptr name)
   (catch #t
-    (lambda () (dynamic-pointer name (dynamic-link)))
+    (lambda ()
+      (dynamic-pointer name
+                       (cond
+                        ((string-contains %host-type "cygwin")
+                         ;; On Cygwin, dynamic-link does not search
+                         ;; recursively into linked DLLs. Thus, one
+                         ;; needs to link to the core C library DLL
+                         ;; explicitly.
+                         (dynamic-link "cygwin1"))
+                        (else
+                         (dynamic-link)))))
     (lambda (k . args)
       (print-exception (current-error-port) #f k args)
       (write "Skipping test.\n" (current-error-port))