Fix DSO linking

Explicitely link against libm, to avoid linking problems:

CCLD   sbc/sbctester
/usr/bin/ld: sbc/sbctester.o: undefined reference to symbol 'sqrt@@GLIBC_2.2.5'
/usr/bin/ld: note: 'sqrt@@GLIBC_2.2.5' is defined in DSO /lib64/libm.so.6 so try adding it to the linker command line
/lib64/libm.so.6: could not read symbols: Invalid operation
diff --git a/Makefile.am b/Makefile.am
index f8b1c11..e55cab3 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -81,7 +81,7 @@
 if SNDFILE
 noinst_PROGRAMS += sbc/sbctester
 
-sbc_sbctester_LDADD = @SNDFILE_LIBS@
+sbc_sbctester_LDADD = @SNDFILE_LIBS@ -lm
 sbc_sbctest_CFLAGS = @SNDFILE_CFLAGS@
 endif
 endif