Problem: missing Ws2_32.lib when cross compiling

Solution: use lower case ws2_32.lib to enable cross compilation
from platforms with case-sensitive filesystems.

When cross compiling the tests with Mingw-w64, CMake cannot
locate Ws2_32.lib

tests/CMakeLists.txt

@@ -134,7 +134,7 @@ if(WIN32)
     add_definitions(-DZMQ_CUSTOM_PLATFORM_HPP)
     add_definitions(-D_WINSOCK_DEPRECATED_NO_WARNINGS)
     # Same name on 64bit systems
-    link_libraries(Ws2_32.lib)
+    link_libraries(ws2_32.lib)
 endif()
 
 # add library and include dirs for all targets