Automate building external libraries #1692

Job	Run time
test_mpi_remote_nexus (ubuntu-latest)	1m 31s
test_bmi_python (ubuntu-latest)	3m 51s
test_bmi_fortran (ubuntu-latest)	4s
test_bmi_python (macos-latest)	4m 54s
test_unit (ubuntu-latest)	4m 53s
test_bmi_fortran (macos-latest)	15s
test_unit (macos-latest)	5m 48s
test_bmi_cpp (ubuntu-latest)	2m 56s
test_bmi_multi (ubuntu-latest)	5s
test_bmi_cpp (macos-latest)	4m 0s
test_pet (ubuntu-latest)	2m 47s
test_bmi_multi (macos-latest)	13s
test_pet (macos-latest)	4m 39s
test_bmi_c (ubuntu-latest)	3m 23s
test_bmi_c (macos-latest)	5m 14s
	44m 33s

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