From https://github.com/mongodb/mongo/pull/166 (Created January 2012)

The macro "LOG" should be removed because the name "LOG" is too common and conflicts with some other products.

For example, current mongodb is not work with google-glog. It also uses "LOG" macro for logging.

Here is a sample code:

#include <glog/logging.h>
#include <client/dbclient.h> // <- undefs LOG macro

int main() {
  LOG(ERROR) << "hoge"; // compile error
}