fwi: Use 4_MPI_ompss variant.

The -D_GNU_SOURCE define is required before mcc includes nanos6.h

bsc/apps/fwi/default.nix

@@ -9,13 +9,16 @@
 
 stdenv.mkDerivation rec {
   name = "nbody";
+  variant = "4_MPI_ompss";
 
   src = builtins.fetchGit {
     url = "https://gitlab.com/srodrb/BSC-FWI.git";
-    ref = "ompss";
+    ref = "ompss-mpi-nocache";
   };
 
-  postUnpack = "sourceRoot=$sourceRoot/3_ompss";
+  postUnpack = "sourceRoot=$sourceRoot/${variant}";
+
+  enableParallelBuilding = true;
 
   buildInputs = [
     nanos6
@@ -25,6 +28,22 @@ stdenv.mkDerivation rec {
     mcxx
   ];
 
+  # FIXME: This is an ugly hack.
+  # When using _GNU_SOURCE or any other definition used in features.h, we need
+  # to define them before mcc includes nanos6.h from the command line. So the
+  # only chance is by setting it at the command line with -D. Using the DEFINES
+  # below, reaches the command line of the preprocessing stage with gcc.
+  preBuild = ''
+    export DEFINES=-D_GNU_SOURCE
+  '';
+
+  makeFlags = [
+    "NZF=108"
+    "NXF=108"
+    "NYF=208"
+    "PRECISION=float"
+  ];
+
   installPhase = ''
     mkdir -p $out/bin
     cp fwi.* $out/bin