Ensure compatibility with a single shared libvips library

See: #372.

lib/vips.rb

@@ -42,7 +42,19 @@ class << self
 
  extend FFI::Library
 
- ffi_lib library_name("glib-2.0", 0)
+ if FFI::Platform.windows?
+ # On Windows, `GetProcAddress()` can only search in a specified DLL and
+ # doesn't look into its dependent libraries for symbols. Therefore, we
+ # check if the GLib DLLs are available. If these can not be found, we
+ # assume that GLib is statically linked into libvips.
+ ffi_lib ['libglib-2.0-0.dll', 'libvips-42.dll']
+ else
+ # macOS and *nix uses `dlsym()`, which also searches for named symbols
+ # in the dependencies of the shared library. Therefore, we can support
+ # a single shared libvips library with all dependencies statically
+ # linked.
+ ffi_lib library_name("vips", 42)
+ end
 
  attach_function :g_malloc, [:size_t], :pointer
 
@@ -134,7 +146,11 @@ def self.set_log_domain domain
 module GObject
  extend FFI::Library
 
- ffi_lib library_name("gobject-2.0", 0)
+ if FFI::Platform.windows?
+ ffi_lib ['libgobject-2.0-0.dll', 'libvips-42.dll']
+ else
+ ffi_lib library_name("vips", 42)
+ end
 
  # we can't just use ulong, windows has different int sizing rules
  if FFI::Platform::ADDRESS_SIZE == 64