Make FindLibraryTest platform-dependent

The original code was added by #7439.
See https://drake-jenkins.csail.mit.edu/view/Continuous%20Production/job/mac-sierra-clang-bazel-continuous-release/271/consoleText

common/test/find_loaded_library_test.cc

@@ -13,7 +13,12 @@ GTEST_TEST(FindLibraryTest, Library) {
   // Test wether or not `LoadedLibraryPath()` can find the path to a library
   // loaded by the process.
   optional<string> library_path =
+  // TODO(fbudin69500): Remove this platform-dependency.
+#ifdef __APPLE__
+    LoadedLibraryPath("libgflags.so");
+#else
     LoadedLibraryPath("libexternal_Scom_Ugithub_Ugflags_Ugflags_Slibgflags.so");
+#endif
   EXPECT_TRUE(library_path);
   library_path = LoadedLibraryPath("lib_not_real.so");
   EXPECT_FALSE(library_path);