Use pkg-config to handle cgo build flags

Hi,

I just made a change in order to use `pkg-config` to handle the various build flags needed by the `cgo` compilation (`CFLAGS`, `LDFLAGS`...).

It helps to build the package with on OSes having `librrd` in a custom location (e.g. rrdtool using `brew` under Mac OS).

Let me know if this PR is ok for you.

Regards