install: Added gcc, build-essential to apt-get command line.

Mercury now requires build tools to install.

R=rsc
CC=golang-dev
https://golang.org/cl/210042

doc/install.html

@@ -186,7 +186,7 @@ <h2>Fetch the repository</h2>
 </pre>
 
 <p>works on most systems.
-(On Ubuntu, you might try <code>apt-get install python-setuptools python-dev</code> first.)
+(On Ubuntu/Debian, you might try <code>apt-get install python-setuptools python-dev build-essential gcc</code> first.)
 If that fails, visit the <a href="http://mercurial.selenic.com/wiki/Download">Mercurial Download</a> page.</p>
 
 <p>Make sure the <code>$GOROOT</code> directory does not exist or is empty.