AutotoolsPackage: minor improvements (#2859)

* AutotoolsPackage: added configure_directory to permit build out of source. The configure script executable is now invoked with an absolute path. Modified a few packages accordingly.

* build_systems: functions returning directories are now properties

* build_systems: fixed issues with tcl and tk

* AutotoolsPackage: reworked recipe for autoreconf

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

@@ -37,5 +37,7 @@ class Libuv(AutotoolsPackage):
     depends_on('libtool', type='build')
 
     def autoreconf(self, spec, prefix):
+        # This is needed because autogen.sh generates on-the-fly
+        # an m4 macro needed during configuration
         bash = which("bash")
         bash('autogen.sh')