Enable configuring the Makefile template using an environment variable

[spencerkclark]

Currently the Makefile template included in the wrapper Makefile is hard coded to be the gnu.mk file from SHiELD_build:

SHiELD-wrapper/wrapper/Makefile

Line 4 in f88d47e

include $(SHIELD_BUILD)/site/gnu.mk

This is fine in Docker, when we are always using GNU compilers, but on other platforms we may want to use different compilers, like Intel. It would be useful if this were optionally configurable with an environment variable, similar to FC, CC, and the like:

SHiELD-wrapper/wrapper/Makefile

Lines 6 to 9 in f88d47e

	FC ?= mpif90
	CC ?= mpicc
	CXX ?= mpicxx
	LD ?= mpif90

We could call this variable MAKEFILE_TEMPLATE, which by default could be set to gnu.mk:

MAKEFILE_TEMPLATE ?= gnu.mk
include $(SHIELD_BUILD)/site/$(MAKEFILE_TEMPLATE)