Just happened to visit this thread after struggling with the same problem.
As commented by @Javier Rivera in the question, the log did say something:
Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: -fstack-protector-strong
not supported by compiler
So the solution is simple: upgrade the GCC suite (at least to 4.9).
For my 14.04 Ubuntu box, the newest one available in official repository is just 4.8, so some hack is needed. Fortunately, this is quite a common problem and is already solved by others:
How to install the latest g++(currently 5.1) in Ubuntu(currently 14.04)?
After installing a newer GCC suite (for me, its 5.3.0), another problem arises when choosing the default compiler for the system, which is also mentioned in the linked guide of the answer to he aforementioned question.
After all of this, double check if you have the right version of gcc:
$ gcc --version
gcc (Ubuntu 5.3.0-3ubuntu1~14.04) 5.3.0 20151204
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Reinstall virtualbox as usual, and this time, it shouldn't complain.