Also add the Gadget-2 value of the constant H_0/h

src/physical_constants_cgs.h

@@ -130,10 +130,20 @@ const double const_T_CMB_0_cgs = 2.7255;
 /*! Primordial Helium fraction (from WMAP7) [-] */
 const double const_primordial_He_fraction_cgs = 0.248;
 
+#ifdef SWIFT_USE_GADGET2_PHYSICAL_CONSTANTS
+
+/*! Reduced Hubble constant units (i.e. H_0 / h == 100 km / s / Mpc in CGS)
+ * [s^-1] */
+const double const_reduced_hubble_cgs = 3.2407789e-18;
+
+#else
+
 /*! Reduced Hubble constant units (i.e. H_0 / h == 100 km / s / Mpc in CGS)
  * [s^-1] */
 const double const_reduced_hubble_cgs = 3.2407792894458e-18;
 
+#endif
+
 /*! Case B recombination coefficient for hydrogen at 10^4 K [cm^3 s^-1] */
 const double const_caseb_recomb_cgs = 2.6e-13;