Build for use within a SQLite3 loadable extension

[mlin]

Greetings @SRombauts or maintainer whom it may concern

I have an interesting use case in which I want to use SQLiteCpp's convenient API within the implementation of a SQLite3 loadable extension. There are three bits needed,

1. Construct a SQLite::Database object to wrap a given sqlite3* connection (passed to us as the extension is loaded) instead of opening a new one. Straightforward, but check my work?
2. The SQLiteCpp object code must be built against a "version" of the SQLite3 API provided by sqlite3ext.h instead of sqlite3.h, with reference to an extern symbol needed to make that work (usually hidden behind the SQLITE_EXTENSION_INIT1 macro). Here activated by the new SQLITECPP_IN_EXTENSION CMake option.
3. The SQLiteCpp library must not itself link SQLite3, since whatever program loads the extension will necessarily already have it resident. In particular, the main program could have SQLite3 statically linked, and extremely confusing things are liable to happen if the extension also dynamically links a different version of SQLite3. Here this is also controlled by SQLITECPP_IN_EXTENSION

With the enclosed example, it all seems to work nicely. Thanks for consideration!

[coveralls]

Coverage remained the same at 100.0% when pulling 6d089fc on mlin:use-in-loadable-extension into 0c46d86 on SRombauts:master.