Automate building external libraries #1691

Job	Run time
test_mpi_remote_nexus (ubuntu-latest)	1m 2s
test_bmi_c (ubuntu-latest)	52s
test_bmi_fortran (ubuntu-latest)	5s
test_bmi_cpp (ubuntu-latest)	52s
test_bmi_c (macos-latest)	1m 57s
test_bmi_fortran (macos-latest)	16s
test_bmi_cpp (macos-latest)	2m 14s
test_pet (ubuntu-latest)	51s
test_pet (macos-latest)	1m 55s
test_unit (ubuntu-latest)	47s
test_unit (macos-latest)	1m 55s
test_bmi_multi (ubuntu-latest)	5s
test_bmi_python (ubuntu-latest)	1m 0s
test_bmi_multi (macos-latest)	14s
test_bmi_python (macos-latest)	2m 47s
	16m 52s

Provide feedback