Minor Description
Rationale:
As a build user I should be able to set the AUTOMAKE and ACLOCAL environment variables for autoconf to pickup when using the bootstrap script.
This is required when you have available in your path an AUTOMAKE that is not compatible with the one that Mesos requires. Currently there is no way you can do this through the bootstrap script.
As an example I should be able to do
```
AUTOMAKE="/usr/local/bin/automake-1.12" ACLOCAL="/usr/local/bin/aclocal-1.12" ./bootstrap
```
