Add support for SYSTEMC_HOME environment variable #40

Job	Run time
Build (2.3.2, ubuntu, 20.04, g++-9, gcc-9)	2m 27s
Build (2.3.2, ubuntu, 20.04, clang++-9, clang-9, clang-9)	2m 26s
Build (2.3.2, ubuntu, 22.04, g++-11, gcc-11)	2m 23s
Build (2.3.2, ubuntu, 22.04, clang++-11, clang-11, clang-11)	3m 15s
Build (2.3.2, macos, 11, g++, gcc, automake)	5m 25s
Build (2.3.2, macos, 11, clang++, clang, automake)	5m 17s
Build (2.3.2, macos, 12, g++, gcc, automake)	3m 13s
Build (2.3.2, macos, 12, clang++, clang, automake)	3m 41s
Build (2.3.3, ubuntu, 20.04, g++-9, gcc-9)	2m 30s
Build (2.3.3, ubuntu, 20.04, clang++-9, clang-9, clang-9)	2m 28s
Build (2.3.3, ubuntu, 22.04, g++-11, gcc-11)	2m 19s
Build (2.3.3, ubuntu, 22.04, clang++-11, clang-11, clang-11)	2m 41s
Build (2.3.3, macos, 11, g++, gcc, automake)	5m 58s
Build (2.3.3, macos, 11, clang++, clang, automake)	6m 24s
Build (2.3.3, macos, 12, g++, gcc, automake)	3m 9s
Build (2.3.3, macos, 12, clang++, clang, automake)	5m 28s
Build (2.3.4, ubuntu, 20.04, g++-9, gcc-9)	2m 38s
Build (2.3.4, ubuntu, 20.04, clang++-9, clang-9, clang-9)	2m 27s
Build (2.3.4, ubuntu, 22.04, g++-11, gcc-11)	2m 27s
Build (2.3.4, ubuntu, 22.04, clang++-11, clang-11, clang-11)	3m 7s
Build (2.3.4, macos, 11, g++, gcc, automake)	1m 6s
Build (2.3.4, macos, 11, clang++, clang, automake)	1m 12s
Build (2.3.4, macos, 12, g++, gcc, automake)	1m 16s
Build (2.3.4, macos, 12, clang++, clang, automake)	1m 7s
	1h 14m 24s

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