Make pkg-config file relocatable

[baranovmv]

Make libdir/incdir entries as prefix-relative paths,
so the .pc can be relocated to a diferent path.
Assert if a path is not inside prefix.

E.g.

10,11c10,11
< Libs: -L/roc/lib -lroc -lrt -ldl -lm -lpthread
< Cflags: -I/roc/include
---
> Libs: -L${prefix}/lib -lroc -lrt -ldl -lm -lpthread
> Cflags: -I${prefix}/include

[rocstreaming-bot]

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