gnurobbo: fix build

srcpkgs/gnurobbo/template

@@ -1,7 +1,7 @@
 # Template file for 'gnurobbo'
 pkgname=gnurobbo
 version=0.68
-revision=2
+revision=3
 build_wrksrc=gnurobbo
 build_style=gnu-makefile
 hostmakedepends="pkg-config"
@@ -15,8 +15,9 @@ checksum=6c7e9e425655da02c2442d13b5fa8f542af62d2c8f253db2fb63470c5c0c3db8
 
 pre_build() {
 	sed -i Makefile \
-		-e "s;CFLAGS\?=.*;CFLAGS=${CFLAGS};" \
-		-e "s;LDFLAGS\?=.*;LDFLAGS=${LDFLAGS};" \
+		-e "s;\(CFLAGS\?+=\)-d;\1;" \
+		-e "s;CFLAGS+\?=;\0 ${CFLAGS} ;" \
+		-e "s;LDFLAGS+\?=;\0 ${LDFLAGS} ;" \
 		-e "/LIBS=/s;\(.*\);\1 -lm;"
 }