Use pkg-config for detecting include and lib dirs

Detect sqlite3 include_dirs and library_dirs using
'pkg-config --cflags-only-I' and 'pkg-config --libs-only-L'
commands respectively.

Currently, it's possible to override that manually
by changing setup.cfg, but it could be inconvenient in some
cases, for example when creating a venv and installing
dependencies using pip from the requirements file.