build: Set no_undefined in Makefile.decl

This simplifies the code and - more importantly - fixes the cases like
in testsuite/reftests where the no_undefined definition was missing.

https://bugzilla.gnome.org/show_bug.cgi?id=736382

modules/printbackends/lpr/Makefile.am

@@ -1,9 +1,5 @@
 include $(top_srcdir)/Makefile.decl
 
-if PLATFORM_WIN32
-no_undefined = -no-undefined
-endif
-
 AM_CPPFLAGS = \
 	-I$(top_srcdir) 				\
 	-I$(top_srcdir)/gtk				\