Makefile.kokkos incompatible with GNU make 4.3

[pgrete]

From https://lists.gnu.org/archive/html/info-gnu/2020-01/msg00004.html

* WARNING: Backward-incompatibility!
  Number signs (#) appearing inside a macro reference or function invocation
  no longer introduce comments and should not be escaped with backslashes:
  thus a call such as:
    foo := $(shell echo '#')
  is legal.  Previously the number sign needed to be escaped, for example:
    foo := $(shell echo '\#')
  Now this latter will resolve to "\#".  If you want to write makefiles
  portable to both versions, assign the number sign to a variable:
    H := \#
    foo := $(shell echo '$H')

The resulting KokkosCore_config.h looks like

/* ---------------------------------------------
Makefile constructed configuration:
Sat 14 Mar 2020 09:44:44 AM CET
----------------------------------------------*/
\#if !defined(KOKKOS_MACROS_HPP) || defined(KOKKOS_CORE_CONFIG_H)
\#error "Do not include KokkosCore_config.h directly; include Kokkos_Macros.hpp instead."
\#else
\#define KOKKOS_CORE_CONFIG_H
\#endif

\#define KOKKOS_VERSION 30000
[...]

and doesn't work anymore.