Automate building external libraries #1712

Job	Run time
test_bmi_c (ubuntu-latest)	3m 18s
test_bmi_cpp (ubuntu-latest)	2m 32s
test_bmi_c (macos-latest)	4m 58s
test_bmi_fortran (ubuntu-latest)	2m 51s
test_bmi_cpp (macos-latest)	5m 24s
test_bmi_fortran (macos-latest)	4m 31s
test_bmi_python (ubuntu-latest)	3m 32s
test_mpi_remote_nexus (ubuntu-latest)	1m 21s
test_bmi_python (macos-latest)	5m 6s
test_pet (ubuntu-latest)	2m 48s
test_pet (macos-latest)	6m 14s
test_unit (ubuntu-latest)	4m 41s
test_bmi_multi (ubuntu-latest)	4m 10s
test_unit (macos-latest)	7m 41s
test_bmi_multi (macos-latest)	4m 41s
	1h 3m 48s

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