Setting up on Linux, some basic questions that googling didn't answer

Linux
12

You need to install the curl development package:

See here:

It will be linked automatically by CodeBlocks.

JUCE_VST3_CAN_REPLACE_VST2 is for doing the opposite - you set this when you require backwards compatible VST3 plug-ins which use VST2 SDK. It’s off by default for new projects.