Remove -Wdeclaration-after-statement as first using -std=gnu99 to allow c99
features and then explicitly requesting warnings about using c99 features
makes little sense.

Remove -W option as it is just backwards compatibility alias for
-Wextra.

Remove -Wformat as it is implied by having -Wall present.
Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>