GDK/Win32: Fix MinGW autotools build

MinGW-w64 CRT provides no 'hid.lib' file. Instead, it has 'libhid.a'
which can be linked with '-lhid' linker argument.

Also, we have to declare the '_LIBADD' variable and add 'LDADDS' to it,
or 'LDADDS' won't do anything for the build.

gdk/win32/Makefile.am

@@ -22,7 +22,7 @@ AM_CPPFLAGS += "-DGDK_WIN32_ENABLE_EGL=1"
 endif #WIN32_GLES
 
 libgdk_win32_DEP_LIBS = \
-	hid.lib
+	-lhid
 
 LDADDS = $(libgdk_win32_DEP_LIBS) $(GDK_DEP_LIBS)
 
@@ -87,6 +87,10 @@ libgdk_win32_la_SOURCES = \
 	wintab.h \
 	xcursors.h
 
+libgdk_win32_la_LIBADD = \
+	$(LDADDS) \
+	$(NULL)
+
 libgdkinclude_HEADERS =		\
 	gdkwin32.h