mongoose: provide libmongoose.a static library

The package provides a webserver and, eventually, a library: this is
what Centos, among other distros, is doing.

[Peter: correct install -D invocation]
Signed-off-by: Davide Viti <d.viti@infosolution.it>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>