sqlite: refactor how CFLAGS are passed to the configure script

Change the way the SQLITE_CONF_ENV variable is constructed in
preparation of passing other CFLAGS to the configure script.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>