libcompat: Always add an empty file to the libcompat static library

Some ar implementations do not support creating empty archives, so we
add a dummy empty object to avoid any problem.

lib/compat/Makefile.am

@@ -8,6 +8,7 @@ AM_CPPFLAGS = \
 noinst_LIBRARIES = libcompat.a
 
 libcompat_a_SOURCES = \
+	empty.c \
 	compat.h \
 	gettext.h
 

lib/compat/empty.c

@@ -0,0 +1 @@
+/* Some implementations of ar cannot create an empty archive. */