libwebsockets

cmake: disable export visibility when building lws static

This is complicated by the fact extern on a function declaration implies
visibility... we have to make LWS_EXTERN empty when building static.

And, setting target_compile_definitions() doesn't work inside macros,
so it has to be set explicitly for the plugins.

Checking the symbol status needs nm -C -D as per

https://stackoverflow.com/questions/37934388/symbol-visibility-not-working-as-expected

after this patch, libwebsockets.a shows no symbols when checked like that and
the static-linked minimal examples only show -U for their other dynamic
imports.

In a handful of cases we use LWS_EXTERN on extern data declarations,
those then need to change to explicit extern.