Automate building external libraries #1697

Job	Run time
test_bmi_cpp (ubuntu-latest)	3m 23s
test_mpi_remote_nexus (ubuntu-latest)	1m 25s
test_bmi_cpp (macos-latest)	3m 56s
test_pet (ubuntu-latest)	2m 37s
test_pet (macos-latest)	5m 40s
test_bmi_c (ubuntu-latest)	3m 33s
test_bmi_fortran (ubuntu-latest)	5s
test_bmi_c (macos-latest)	4m 22s
test_bmi_fortran (macos-latest)	15s
test_bmi_multi (ubuntu-latest)	6s
test_bmi_multi (macos-latest)	52s
test_bmi_python (ubuntu-latest)	3m 33s
test_unit (ubuntu-latest)	4m 33s
test_unit (macos-latest)	8m 15s
test_bmi_python (macos-latest)	6m 34s
	49m 9s

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