This example is for Mac but the same principles apply to other targets.
Add the library name to the “External libraries to link” property in the target:
In this case I was using curl, the library file is called libcurl.a — so in terms of what you enter into the Projucer strip off “lib” from the the front and remove the file extension.
And add the search paths for the headers and the lib in each of the targets configurations (Debug and Release):
If some of your files pull the header at #include <libusb.h> and some #include <libusb-1.0/libusb.h> then you might need to add both of these to your header search paths:
/usr/include/libusb-1.0
/usr/include
Alternatively you could add the full path of the library (including the lib prefix and the file extension) to the “Extra linker flags” section of the target properties:
But you’ll still need to provide header seach paths for the code to compile.