bmon: link dynamically unless explicitly requested otherwise

Commit 009d8fce (add --{enable,disable}-{shared,static} automatically)
caused us to pass --enable-static in the normal (non-static) case as well,
which bmon interpretes as a request to link the bmon program statically,
bloating the image.

Fix it by passing an extra --disable-static to bmon configure to nullify
this.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>