Add support for out-of-source build and auto-generated dependencies

[hutorny]

This PR suggests

1. supporting for Out-of-source build via optional, externally provided BUILDDIR variable.
2. separating installation paths from the include/library search paths to enable staged builds

Usage example:

make -j$(nproc) BUILDDIR=/tmp/wdsp
sudo make install BUILDDIR=/tmp/wdsp