Detect external libiconv and make libiconv and git non-buildable on macOS

[climbfuji]

Summary

Detect external libiconv and make libiconv and git non-buildable on macOS. This addresses the issue mentioned in #672 (comment).

Making git and libiconv non-buildable on macOS avoids running these commands manually as users.

Testing

Tested on my macOS laptop.

Applications affected

n/a

Systems affected

macOS systems

Dependencies

• waiting on Detect external libiconv on macOS, make non-buildable spack#301

Issue(s) addressed

See #672 (comment)

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.

[srherbener]

Thanks for this update! Tested jedi-bundle on my Mac with this build. All expected tests passed.

[climbfuji]

Thanks @srherbener. I got the following ctest failures for jedi-bundle when using Rosetta2 x86_64. I think these are expected.

The following tests FAILED:
	 14 - test_util_signal_trap (Failed)
	355 - saber_test_error_covariance_training_bump_hdiag-nicas_2_1-1 (Failed)
	440 - saber_test_error_covariance_training_bump_hdiag-nicas_2_2-1 (Failed)
	1087 - ufo_test_tier1_test_ufo_qc_variableassignment (Failed)
	1211 - ufo_test_tier1_test_ufo_opr_gnssrorefmetoffice (Failed)
	1215 - ufo_test_tier1_test_ufo_opr_gnssrobendmetoffice_nopseudo (Failed)
	1223 - ufo_test_tier1_test_ufo_opr_groundgnssmetoffice (Failed)
	1276 - ufo_test_tier1_test_ufo_opr_scatwind_neutral_metoffice (Failed)
	1450 - fv3jedi_test_tier1_errorcovariance (Failed)
	1496 - fv3jedi_test_tier1_errorcovariance_bump (Failed)
	1559 - test_soca_errorcovariance (Failed)

Also, on the same machine using the native arch64 build, I got the following ctest failures (expected?):

The following tests FAILED:
	 14 - test_util_signal_trap (Failed)
	355 - saber_test_error_covariance_training_bump_hdiag-nicas_2_1-1 (Failed)
	440 - saber_test_error_covariance_training_bump_hdiag-nicas_2_2-1 (Failed)
	745 - test_ioda-engines-00-Strings-F (SEGFAULT)
	1071 - ufo_test_tier1_test_ufo_qc_average_obs_to_mod_levels (Failed)
	1087 - ufo_test_tier1_test_ufo_qc_variableassignment (Failed)
	1211 - ufo_test_tier1_test_ufo_opr_gnssrorefmetoffice (Failed)
	1215 - ufo_test_tier1_test_ufo_opr_gnssrobendmetoffice_nopseudo (Failed)
	1223 - ufo_test_tier1_test_ufo_opr_groundgnssmetoffice (Failed)
	1276 - ufo_test_tier1_test_ufo_opr_scatwind_neutral_metoffice (Failed)
	1450 - fv3jedi_test_tier1_errorcovariance (Failed)
	1496 - fv3jedi_test_tier1_errorcovariance_bump (Failed)
	1560 - test_soca_errorcovariance (Failed)
	1595 - test_soca_sqrtvertloc (Failed)
	1622 - test_soca_convertincrement (Failed)

I'll go ahead and merge the spack PR.

[srherbener]

@climbfuji your list of test failures on the ARM based Mac looks like the one I get. I have one additional test failure on my Mac: ufo_test_tier1_test_ufo_function_orbitangle_ssmis which is due to a breech of a tolerance limit on a floating point value check.