root: enable mathmore

The `mathmore` part of ROOT it the part of ROOT math that depends on the
GNU Scientific Library (GSL). Since the GSL is already in the list of
inputs, it should be harmless to enable this feature set. This will
benefit RooFit in particular, wich uses the numeric integrators from the
GSL that are wrapped in mathmore.

pkgs/applications/science/misc/root/default.nix

@@ -165,6 +165,7 @@ stdenv.mkDerivation rec {
     "-Dfail-on-missing=ON"
     "-Dfitsio=OFF"
     "-Dgnuinstall=ON"
+    "-Dmathmore=ON"
     "-Dmysql=OFF"
     "-Dpgsql=OFF"
     "-Dsqlite=OFF"