Fix SQLite support for CMake users.

Fix SQLite support for CMake users.

FindPkgConfig obviously uses pkg-config's output for setting convenient
variables such as <PREFIX>_LIBRARIES or <PREFIX>_INCLUDE_DIRS. It also
sets <PREFIX>_FOUND to 1 if <PREFIX> module exists.

So why checking for SQLITE3_FOUND is better than (SQLITE3_LIBRARIES AND
SQLITE3_INCLUDE_DIRS)? Apart from obvious readability factor, latter
condition has strong assumption that both variables are filled with
appropriate paths, which is unjustifiable unless you add another
assumptions...

pkg-config by default strips -I/usr/include from Cflags and -L/usr/lib
from Libs if some environment variables are not set,
PKG_CONFIG_ALLOW_SYSTEM_CFLAGS and PKG_CONFIG_ALLOW_SYSTEM_LIBS
respectively. This behavior is sane, because it prevents polluting the
compilation and linking commands with superfluous entries.

In debian SQLITE3_INCLUDE_DIRS is empty for instance.

Remark for developers:
Always check commands invoked by CMake after changing CMakeLists.txt.

    VERBOSE=1 cmake --build .