Use Gentoo's system copy of libbs2b instead of the bundled source.

--- a/Makefile
+++ b/Makefile
@@ -1,18 +1,16 @@
 OUT?=ddb_bs2b.so
 
-CFLAGS+=-std=c99 -fPIC -Wall -I./libbs2b-3.1.0/src
+BS2B_LIBS?=-lbs2b
+CFLAGS+=-std=c99 -fPIC -Wall
 
-BS2B_SOURCES=\
-libbs2b-3.1.0/src/bs2b.c
-
-SOURCES=bs2b.c $(BS2B_SOURCES)
+SOURCES=bs2b.c
 
 OBJECTS=$(SOURCES:.c=.o)
 
 all: $(SOURCES) $(OUT)
 
 $(OUT): $(OBJECTS)
-	$(CC) $(CFLAGS) -shared -lm $(OBJECTS) -o $@ $(LDFLAGS)
+	$(CC) $(LDFLAGS) -shared $(OBJECTS) -o $@ $(BS2B_LIBS) -lm
 
 .c.o:
 	$(CC) $(CFLAGS) $< -c -o $@
--- a/bs2b.c
+++ b/bs2b.c
@@ -8,7 +8,7 @@
 */
 
 #include <deadbeef/deadbeef.h>
-#include "bs2b.h"
+#include <bs2b/bs2b.h>
 #include <stdlib.h>
 #include <string.h>