Use proper gnutls_free() on Windows

The underlying issue requiring this hack was fixed ages ago.
Unfortunately, the fixed GnuTLS doesn't consider static linking. So we
need to add a new hack that permits that.

(cherry picked from commit cfa4cc7)

common/rfb/CMakeLists.txt

@@ -101,6 +101,10 @@ if(GNUTLS_FOUND)
   target_sources(rfb PRIVATE CSecurityTLS.cxx SSecurityTLS.cxx)
   target_include_directories(rfb SYSTEM PUBLIC ${GNUTLS_INCLUDE_DIR})
   target_link_libraries(rfb ${GNUTLS_LIBRARIES})
+  # FIXME: Hack to block it marking gnutls_free() as dllimport
+  if(WIN32 AND BUILD_STATIC)
+    target_compile_definitions(rfb PRIVATE GNUTLS_INTERNAL_BUILD)
+  endif()
 endif()
 
 if (NETTLE_FOUND)

common/rfb/CSecurityTLS.cxx

@@ -46,16 +46,6 @@
 
 #include <gnutls/x509.h>
 
-/*
- * GNUTLS doesn't correctly export gnutls_free symbol which is
- * a function pointer. Linking with Visual Studio 2008 Express will
- * fail when you call gnutls_free().
- */
-#if WIN32
-#undef gnutls_free
-#define gnutls_free free
-#endif
-
 using namespace rfb;
 
 static const char* configdirfn(const char* fn);