Make jpeg a virtual dependency. (#5190)

* Make jpeg a virtual dependency.

* Make 'libjpeg-turbo' the default implementation of 'jpeg'.

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

@@ -35,9 +35,6 @@ class Libtiff(AutotoolsPackage):
     version('4.0.6', 'd1d2e940dea0b5ad435f21f03d96dd72')
     version('4.0.3', '051c1068e6a0627f461948c365290410')
 
-    variant('turbo', default=False, description='use libjpeg-turbo')
-
-    depends_on('jpeg', when='-turbo')
-    depends_on('libjpeg-turbo', when='+turbo')
+    depends_on('jpeg')
     depends_on('zlib')
     depends_on('xz')