See comment in libsqlite3-sys/build.rs for details - adding this flag is
harmless if it's not present in the header, and not having it can break
builds against older SQLite versions.
* If SQLITE3_LIB_DIR is present in the environment, we use that.
* If SQLITE3_LIB_DIR is not present, we try to use pkg-config.
* If SQLITE3_LIB_DIR is not present and pkg-config fails, we fall back
to /usr/lib (if it exists).