"sqlite3.c" and "sqlite3.h" files from sqlite-amalgamation-3120200.zip (SQLite 3.12.2 2016-04-18)
Those files are provided for easy setup and compatibility under Windows/Linux/MacOS. They are used by default by the CMake build.
Use -DSQLITECPP_INTERNAL_SQLITE=OFF to link against the Linux "libsqlite3-dev" package instead.
All of the code and documentation in SQLite has been dedicated to the public domain by the authors.