darwin: explicitly use the 'when=' kwarg

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

@@ -39,7 +39,7 @@ class Glib(Package):
     depends_on("libffi")
     depends_on("zlib")
     depends_on("pkg-config")
-    depends_on('gettext', sys.platform == 'darwin')
+    depends_on('gettext', when=sys.platform == 'darwin')
 
     # The following patch is needed for gcc-6.1
     patch('g_date_strftime.patch')

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

@@ -35,7 +35,7 @@ class Numdiff(Package):
 
     version('5.8.1',    'a295eb391f6cb1578209fc6b4f9d994e')
 
-    depends_on('gettext', sys.platform=='darwin')
+    depends_on('gettext', when=sys.platform=='darwin')
 
     def install(self, spec, prefix):
         options = ['--prefix=%s' % prefix]

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

@@ -68,7 +68,7 @@ class Octave(Package):
     depends_on('blas')
     depends_on('lapack')
     # Octave does not configure with sed from darwin:
-    depends_on('sed', sys.platform == 'darwin')
+    depends_on('sed', when=sys.platform == 'darwin')
     depends_on('pcre')
     depends_on('pkg-config')