meson: fix symbol exporting on simultaneous static and shared building

[antoniovazquezblanco]

It turns out #46 was not the best solution.

default_library is a tri-state. When both libraries are being built, it is important to also set the exported flag.

[likle]

Hi @antoniovazquezblanco ! I am not verify familiar with meson, doesn't "both" mean it builds shared as well as static?

Wouldn't that set it to CWK_SHARED for the static build done using the both option?

[antoniovazquezblanco]

Let's move the conversation to mesonbuild/wrapdb#1387, those guys know more than me :D