Automate building external libraries #1702

Job	Run time
test_mpi_remote_nexus (ubuntu-latest)	1m 35s
test_bmi_cpp (ubuntu-latest)	2m 32s
test_bmi_cpp (macos-latest)	4m 0s
test_bmi_multi (ubuntu-latest)	7s
test_bmi_python (ubuntu-latest)	3m 36s
test_bmi_c (ubuntu-latest)	3m 40s
test_bmi_python (macos-latest)	6m 23s
test_bmi_fortran (ubuntu-latest)	2m 48s
test_bmi_c (macos-latest)	4m 36s
test_bmi_fortran (macos-latest)	2m 8s
test_bmi_multi (macos-latest)	16s
test_pet (ubuntu-latest)	2m 32s
test_unit (ubuntu-latest)	4m 41s
test_pet (macos-latest)	5m 29s
test_unit (macos-latest)	9m 10s
	53m 33s

Provide feedback