global

def library_name(name, abi_number)

"lib" prefix or a ".dll" suffix.
The ABI number must be included, but with a hyphen. ffi does not add a
windows:
As linux, but the extension is at the end and is added by ffi.
mac:
prefix.
Confusingly, the file extension is not at the end. ffi adds the "lib"
installed. To work everywhere, you must include the ABI number.
Some distros allow "libvips.so", but only if the -dev headers have been
linux:
Platform notes:

Generate a library name for ffi. 
def library_name(name, abi_number)
  if FFI::Platform.windows?
    "lib#{name}-#{abi_number}.dll"
  elsif FFI::Platform.mac?
    "#{name}.#{abi_number}"
  else
    "#{name}.so.#{abi_number}"
  end
end