Only link against libessentia in the pc file. Fixes #340

[alastair]

When we want to link libessentia.so into a program we only need to specify -lessentia. Other dependencies of essentia are already correctly linked against the .so. If a user wants to also link their program against the same dependency (e.g. libav) then they should explicitly link against that themselves.

[dbogdanov]

Those dependencies are required for the static version of the library. Probably two different .pc files can be generated depending on whether the shared or static libessentia was built.

[sildeag]

One thing to think about is the location of the.pc files. On Ubuntu, to the best of my understanding, /usr/local/share/pkgconfig is automatically searched. /usr/local/lib/pkgconfig may be on the automatic search path in many other flavours of linux but requires PKG_CONFIG_PATH to be set to it on Ubuntu.

[dbogdanov]

The correct way would be to specify Essentia's dependencies in Libs.private when generating the pc file.

@sildeag Here is some related info: /usr/share is for architecture-independent data, for that reason using /usr/local/lib/pkgconfig is more appropriate.

[sildeag]

@dbogdanov thanks for the info. I was just pointing out that /usr/local/lib/pkgconfig wasn't in the automatic search path for Ubuntu. This impacts build scripts which use .pc files - since on Ubuntu not everyone has PKG_CONFIG_PATH set and/or has some out-dated .pc file on their system that is in the automatic search path.