GNU/Hurd platform support fixes (#9873)

Define use of -rpath-link and --export-dynamic flags.  The toolchain on
Hurd is GNU-based so we can use the values from Linux.

Modules/Platform/GNU.cmake

@@ -4,5 +4,8 @@ SET(CMAKE_SHARED_LIBRARY_C_FLAGS "-fPIC")
 SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-shared")
 SET(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG "-Wl,-rpath,")
 SET(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG_SEP ":")
+SET(CMAKE_SHARED_LIBRARY_RPATH_LINK_C_FLAG "-Wl,-rpath-link,")
 SET(CMAKE_SHARED_LIBRARY_SONAME_C_FLAG "-Wl,-soname,")
+SET(CMAKE_EXE_EXPORTS_C_FLAG "-Wl,--export-dynamic")
+
 INCLUDE(Platform/UnixPaths)