Add support for SYSTEMC_HOME environment variable #43

Job	Run time
Build (2.3.2, ubuntu, 20.04, g++-9, gcc-9)	1m 20s
Build (2.3.2, ubuntu, 20.04, clang++-9, clang-9, clang-9)	1m 28s
Build (2.3.2, ubuntu, 22.04, g++-11, gcc-11)	1m 25s
Build (2.3.2, ubuntu, 22.04, clang++-11, clang-11, clang-11)	1m 32s
Build (2.3.2, macos, 11, g++, gcc, automake)	3m 7s
Build (2.3.2, macos, 11, clang++, clang, automake)	2m 50s
Build (2.3.2, macos, 12, g++, gcc, automake)	1m 41s
Build (2.3.2, macos, 12, clang++, clang, automake)	1m 41s
Build (2.3.3, ubuntu, 20.04, g++-9, gcc-9)	1m 23s
Build (2.3.3, ubuntu, 20.04, clang++-9, clang-9, clang-9)	1m 23s
Build (2.3.3, ubuntu, 22.04, g++-11, gcc-11)	1m 27s
Build (2.3.3, ubuntu, 22.04, clang++-11, clang-11, clang-11)	1m 28s
Build (2.3.3, macos, 11, g++, gcc, automake)	3m 4s
Build (2.3.3, macos, 11, clang++, clang, automake)	3m 1s
Build (2.3.3, macos, 12, g++, gcc, automake)	2m 44s
Build (2.3.3, macos, 12, clang++, clang, automake)	4m 8s
Build (2.3.4, ubuntu, 20.04, g++-9, gcc-9)	1m 24s
Build (2.3.4, ubuntu, 20.04, clang++-9, clang-9, clang-9)	1m 25s
Build (2.3.4, ubuntu, 22.04, g++-11, gcc-11)	1m 20s
Build (2.3.4, ubuntu, 22.04, clang++-11, clang-11, clang-11)	1m 26s
Build (2.3.4, macos, 11, g++, gcc, automake)	1m 9s
Build (2.3.4, macos, 11, clang++, clang, automake)	1m 7s
Build (2.3.4, macos, 12, g++, gcc, automake)	1m 17s
Build (2.3.4, macos, 12, clang++, clang, automake)	54s
	43m 44s

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