Imporve FFT error message

src/Spectral.cc

@@ -299,7 +299,6 @@ public:
                 if (numMissVals) cdo_abort("Missing values unsupported for spectral data!");
 
                 gridID1 = varList1.vars[varID].gridID;
-                ;
                 // clang-format off
                 if      (lgp2sp) grid2spec(spTrans, gridID1, array1, gridID2, array2);
                 else if (lsp2gp) spec2grid(spTrans, gridID1, array1, gridID2, array2);

src/specspace.h

@@ -74,7 +74,16 @@ public:
       {
         vtrig.resize(nlon);
         auto status = fft_set(vtrig.data(), ifax, nlon);
-        if (status < 0) cdo_abort("FFT error!");
+        if (status < 0)
+          {
+            cdo_print("Retry it with the fftw3 library!");
+#ifdef HAVE_LIBFFTW3
+            cdo_warning("Using fftw3 lib is disabled!");
+#else
+            cdo_warning("LIBFFTW3 support not compiled in!");
+#endif
+            cdo_abort("FFT error!");
+          }
       }
   }
 };