MAINT: Charliecloud OSX error (#16049)

* MAINT: Charliecloud OSX error

* raise an appropriate error when attempting to build
Charliecloud on Mac OSX, since it will otherwise fail
with a more confusing configure stage link check failure

* Update var/spack/repos/builtin/packages/charliecloud/package.py

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* MAINT: PR 16049 revision

* remove an unused import

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

var/spack/repos/builtin/packages/charliecloud/package.py

@@ -33,6 +33,8 @@ class Charliecloud(AutotoolsPackage):
     depends_on('py-sphinx',           type='build', when='+docs')
     depends_on('py-sphinx-rtd-theme', type='build', when='+docs')
 
+    conflicts('platform=darwin', msg='This package does not build on macOS')
+
     # bash automated testing harness (bats)
     depends_on('bats@0.4.0', type='test')