cryptsetup package: dont vendor libuuid in util-linux (#12839)

Fixes #12829

This adds a variant to the util-linux package that controls whether it
builds its own libuuid.  Variant defaults to True.  It enables other
packages to choose to get libuuid from the libuuid package instead.

This also changes the cryptsetup package to build util-linux with
~libuuid (so it uses an explicitly-Spack-built instance of libuuid
instead).

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

@@ -20,7 +20,7 @@ class Cryptsetup(AutotoolsPackage):
     depends_on('lvm2', type=('build', 'link'))
     depends_on('popt', type=('build', 'link'))
     depends_on('json-c', type=('build', 'link'))
-    depends_on('util-linux', type=('build', 'link'))
+    depends_on('util-linux~libuuid', type=('build', 'link'))
     depends_on('gettext', type=('build', 'link'))
 
     depends_on('autoconf', type='build')

var/spack/repos/builtin/packages/util-linux/package.py

@@ -21,9 +21,17 @@ class UtilLinux(AutotoolsPackage):
     depends_on('python@2.7:')
     depends_on('pkgconfig')
 
+    # Make it possible to disable util-linux's libuuid so that you may
+    # reliably depend_on(`libuuid`).
+    variant('libuuid', default=True, description='Build libuuid')
+
     def url_for_version(self, version):
         url = "https://www.kernel.org/pub/linux/utils/util-linux/v{0}/util-linux-{1}.tar.gz"
         return url.format(version.up_to(2), version)
 
     def configure_args(self):
-        return ['--disable-use-tty-group']
+        config_args = [
+            '--disable-use-tty-group',
+        ]
+        config_args.extend(self.enable_or_disable('libuuid'))
+        return config_args