Automate building external libraries #1736

Job	Run time
test_bmi_c (ubuntu-latest)	3m 32s
test_bmi_fortran (ubuntu-latest)	2m 56s
test_bmi_c (macos-latest)	6m 12s
test_bmi_fortran (macos-latest)	6m 26s
test_bmi_python (ubuntu-latest)	3m 41s
test_bmi_python (macos-latest)	7m 42s
test_mpi_remote_nexus (ubuntu-latest)	1m 23s
test_pet (ubuntu-latest)	2m 37s
test_pet (macos-latest)	4m 6s
test_unit (ubuntu-latest)	4m 33s
test_unit (macos-latest)	8m 9s
test_bmi_cpp (ubuntu-latest)	2m 36s
test_bmi_multi (ubuntu-latest)	3m 50s
test_bmi_cpp (macos-latest)	4m 51s
test_bmi_multi (macos-latest)	8m 19s
	1h 10m 53s

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