Added spack option to the build script 

	1. The build script takes spack as an option now. This
	enables building with spack environment variables
	2. conv-mach-spack.sh file contains the environment
	variables to be set and uses the spack variables
	3. Makefile includes the spack_env.sh file created by
	the build script in the bin folder if building from spack
	4. The charmc script includes the spack_env.sh file if
	it is present.
# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch charm
# Your branch is up-to-date with 'origin/charm'.
#
# Changes to be committed:
#	modified:   build
#	new file:   src/arch/common/conv-mach-spack.h
#	new file:   src/arch/common/conv-mach-spack.sh
#	modified:   src/scripts/Makefile
#	modified:   src/scripts/charmc
#
