Fixing build with --as-needed

http://bugs.gentoo.org/294177

Patch written by Kacper Kowalik <xarthisius.kk@gmail.com>

--- src/Makefile.am
+++ src/Makefile.am
@@ -42,4 +42,4 @@
 	openyahtzee.rc
 
 AM_CXXFLAGS = @WX_CXXFLAGS@
-openyahtzee_LDFLAGS = @WX_LIBS@
+openyahtzee_LDADD = @WX_LIBS@