Unable to locate package libX11-dev

I was trying to build HTK using Ubuntu 11.04.

These are the commands I used:

sudo ./configure 
make all

I immediately get the error:

X11/Xlib.h: No such file or directory

So when I try to install libX11-dev by using:

sudo apt-get install libX11-dev

I got the following error:

Unable to locate package libX11-dev

I'd appreciate some help on this.


That's because the file is named libx11-dev

synaptic would be a good thing to have installed & to use for such activities