Improves support for building Google Test as Windows DLL.

src/gtest_main.cc

@@ -31,7 +31,7 @@
 
 #include <gtest/gtest.h>
 
-int main(int argc, char **argv) {
+GTEST_API_ int main(int argc, char **argv) {
   std::cout << "Running main() from gtest_main.cc\n";
 
   testing::InitGoogleTest(&argc, argv);