Fix LTO test on Cygwin

This partially reverts commit add502c648.

In 'linkshared' test, annotate cppfunc() as imported, so an indirection
through an import stub is generated, avoiding a relocation size error
when building using gcc for Cygwin with LTO on.

Align with the example of how to write this portably in [1].

The 'c' language part of that test already gets this right.

[1] http://gcc.gnu.org/wiki/Visibility

run_unittests.py

@@ -3075,8 +3075,6 @@ class AllPlatformTests(BasePlatformTests):
 
     @skip_if_not_base_option('b_lto_threads')
     def test_lto_threads(self):
-        if is_cygwin():
-            raise unittest.SkipTest('LTO is broken on Cygwin.')
         testdir = os.path.join(self.common_test_dir, '6 linkshared')
 
         env = get_fake_env(testdir, self.builddir, self.prefix)

test cases/common/6 linkshared/cpplib.cpp

@@ -1,8 +1,5 @@
-#if defined _WIN32
-    #define DLL_PUBLIC __declspec(dllexport)
-#else
-    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
-#endif
+#define BUILDING_DLL
+#include "cpplib.h"
 
 int DLL_PUBLIC cppfunc(void) {
     return 42;

test cases/common/6 linkshared/cpplib.h

@@ -0,0 +1,12 @@
+/* See http://gcc.gnu.org/wiki/Visibility#How_to_use_the_new_C.2B-.2B-_visibility_support */
+#if defined(_WIN32) || defined(__CYGWIN__)
+  #ifdef BUILDING_DLL
+    #define DLL_PUBLIC __declspec(dllexport)
+  #else
+    #define DLL_PUBLIC __declspec(dllimport)
+  #endif
+#else
+    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
+#endif
+
+int DLL_PUBLIC cppfunc(void);

test cases/common/6 linkshared/cppmain.cpp

@@ -1,4 +1,4 @@
-int cppfunc(void);
+#include "cpplib.h"
 
 int main(void) {
     return cppfunc() != 42;