External headers of C libraries must be linked according to the C convention

src/filt_seis.cpp

@@ -25,7 +25,9 @@
 
 #include "fd.hpp"
 #include "util.hpp"
+extern "C" {
 #include "cseife.h"
+}
 
 void filt_seis(float **data, int ntr, int NS, float finv, const GlobVar *gv)
 {