Wasm compatible fixes

[carlopi]

This is on top of #274, that in some version I will assume will go in.

3 changes are:

• fix build_loadable_extension (currently borked / unusable, even in native)
• feature select src/aws.cpp depending on EMSCRIPTEN define
• feature select extension_config.cmake to skip other extensions in wasm

First makes sense in any case, other 2 should have no influence on native compilation.

[carlopi]

This is ready on my side, not really urgent, whenever.

[Tmonster]

lgtm