Unfortunately previous commit was only a partial fix, because it broke
SQLite support on platforms w/o pkg-config, e.g. Windows. To be honest
I just forgot about messy Windows.

Now if there is no pkg-config, then user must provide two variables:
SQLITE3_INCLUDE_DIRS and SQLITE3_LIBRARIES if (s)he wants to use SQLite
backend. These variables are added to cmake-gui for her/his convenience
unless they are set by FindPkgConfig module.

pkg-config should work also now in Cygwin.