BUILD: Cleaner dl library name (now using a variable)

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
BUILD: Fixed missing flag when building plugin on Linux

When building the VST plugin on Linux the flag LINUX is not passed
to the compiler. We have to do it explicitly from the CMake build system.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>