Why aren't we getting the include dirs from the pc file as well?. In my installation (libeigen3-dev 3.0.5-1), `/usr/share/pkgconfig/eigen3.pc` looks like this:

```
Name: Eigen3
Description: A C++ template library for linear algebra: vectors, matrices, and related algorithms
Requires:
Version: 3.0.5
Libs:
Cflags: -I/usr/include/eigen3
```