Use cups-config --ldflags

This fixes link problems on NixOS.

Makefile.am

@@ -29,6 +29,7 @@ src_rastertobrlaser_SOURCES = \
 	src/line.h \
 	src/line.cc
 src_rastertobrlaser_LDADD = $(CUPS_LIBS)
+src_rastertobrlaser_LDFLAGS = $(CUPS_LDFLAGS)
 
 src_brdecode_SOURCES = \
 	src/brdecode.cc

configure.ac

@@ -38,10 +38,12 @@ AS_IF([test -z "$CUPS_CONFIG"],
   [AC_MSG_ERROR(["cups-config" command not found. Please install the CUPS development package.])])
 CUPS_CFLAGS=`"$CUPS_CONFIG" --cflags`
 CUPS_LIBS=`"$CUPS_CONFIG" --image --libs`
+CUPS_LDFLAGS=`"$CUPS_CONFIG" --image --ldflags`
 CUPS_SERVERBIN=`"$CUPS_CONFIG" --serverbin`
 CUPS_DATADIR=`"$CUPS_CONFIG" --datadir`
 AC_SUBST(CUPS_CFLAGS)
 AC_SUBST(CUPS_LIBS)
+AC_SUBST(CUPS_LDFLAGS)
 AC_SUBST(CUPS_SERVERBIN)
 AC_SUBST(CUPS_DATADIR)