Add a .pc file to NTL

[kiwifb]

Adding a .pc file for NTL helps downstream applications and library to properly find the library and headers in a standard way.

[victorshoup]

@kiwifb

Thanks for the PR! This looks quite reasonable. SInce this generates the file ntl.pc, I wonder if this should also be coordinated with the .gitignore PR.

[kiwifb]

It definitely should! Let me have a look at the PR.

[kiwifb]

OK, two ways of going about that:

1. you merge the .gitignore PR, I rebase on it and add the bits for .pc
2. You ask Matthias to add bits for .pc independently

Either way one of use will have to do a bit of extra work.