Add support for SYSTEMC_HOME environment variable #44

Job	Run time
Build (2.3.2, ubuntu, 20.04, g++-9, gcc-9)	1m 24s
Build (2.3.2, ubuntu, 20.04, clang++-9, clang-9, clang-9)	1m 23s
Build (2.3.2, ubuntu, 22.04, g++-11, gcc-11)	1m 22s
Build (2.3.2, ubuntu, 22.04, clang++-11, clang-11, clang-11)	1m 33s
Build (2.3.2, macos, 11, g++, gcc, automake)	2m 51s
Build (2.3.2, macos, 11, clang++, clang, automake)	3m 7s
Build (2.3.2, macos, 12, g++, gcc, automake)	2m 46s
Build (2.3.2, macos, 12, clang++, clang, automake)	1m 40s
Build (2.3.3, ubuntu, 20.04, g++-9, gcc-9)	1m 21s
Build (2.3.3, ubuntu, 20.04, clang++-9, clang-9, clang-9)	1m 30s
Build (2.3.3, ubuntu, 22.04, g++-11, gcc-11)	1m 19s
Build (2.3.3, ubuntu, 22.04, clang++-11, clang-11, clang-11)	1m 32s
Build (2.3.3, macos, 11, g++, gcc, automake)	2m 52s
Build (2.3.3, macos, 11, clang++, clang, automake)	3m 13s
Build (2.3.3, macos, 12, g++, gcc, automake)	1m 42s
Build (2.3.3, macos, 12, clang++, clang, automake)	2m 37s
Build (2.3.4, ubuntu, 20.04, g++-9, gcc-9)	1m 50s
Build (2.3.4, ubuntu, 20.04, clang++-9, clang-9, clang-9)	1m 49s
Build (2.3.4, ubuntu, 22.04, g++-11, gcc-11)	1m 26s
Build (2.3.4, ubuntu, 22.04, clang++-11, clang-11, clang-11)	1m 25s
Build (2.3.4, macos, 11, g++, gcc, automake)	1m 12s
Build (2.3.4, macos, 11, clang++, clang, automake)	1m 5s
Build (2.3.4, macos, 12, g++, gcc, automake)	1m 10s
Build (2.3.4, macos, 12, clang++, clang, automake)	1m 8s
	43m 17s

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