GenerateExportHeader: Use dllexport/dllimport on Cygwin

Cygwin sits on top of Windows and so can use explicit symbol
export and import markup too.

Co-Author: Yaakov Selkowitz <yselkowitz@cygwin.com>

Modules/GenerateExportHeader.cmake

@@ -268,7 +268,7 @@ macro(_DO_SET_MACRO_VALUES TARGET_LIBRARY)
   get_property(type TARGET ${TARGET_LIBRARY} PROPERTY TYPE)
 
   if(NOT ${type} STREQUAL "STATIC_LIBRARY")
-    if(WIN32)
+    if(WIN32 OR CYGWIN)
       set(DEFINE_EXPORT "__declspec(dllexport)")
       set(DEFINE_IMPORT "__declspec(dllimport)")
     elseif(COMPILER_HAS_HIDDEN_VISIBILITY AND USE_COMPILER_HIDDEN_VISIBILITY)

Tests/Module/GenerateExportHeader/CMakeLists.txt

@@ -106,10 +106,10 @@ endif()
 add_executable(GenerateExportHeader exportheader_test.cpp)
 
 target_link_libraries(GenerateExportHeader ${link_libraries})
-if (WIN32)
+if (WIN32 OR CYGWIN)
   if(MSVC AND COMPILER_HAS_DEPRECATED)
     set(_platform Win32)
-  elseif(MINGW AND COMPILER_HAS_DEPRECATED)
+  elseif((MINGW OR CYGWIN) AND COMPILER_HAS_DEPRECATED)
     set(_platform MinGW)
   else()
     set(_platform WinEmpty)