Fix cgo flags

[dubo-dubon-duponey]

Suggestion to fix #22

This is replacing hard-coded compiler/linker flags in the source code of the project with pkg-config sourced info - works fine on mac and debian derivatives by default for me, since they all provide pc files for libfdk-aac with the appropriate include and lib locations.

If both the static and dynamic versions exist in the system, this will pick-up the dynamic one by default (unless the whole build is -static). If ONLY the static version exist, the static version will be picked-up in all cases without any modification of the go source code.

If one wants to force static linking of JUST fdk-aac, when both dyn and static versions are available on the system, one can manually edit the lib pc file.

As for what should happen in the dockerfile / future binary distribution of this project, I do believe it should be built fully statically (possibly including glibc - but more on that later with an updated dockerfile).

@AlbanSeurat let me know if you think this is an acceptable trade-off.
I do believe the upsides are:

• build will now work by default on all supported platforms, including anything with a non-standard location, as long as PKG_CONFIG_PATH is pointed in the right direction
• picking your flavor (static vs. dynamic) is no longer a matter of changing go code and is properly delegated to the system