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/flex/package.py

@@ -61,11 +61,3 @@ def url_for_version(self, version):
             url += "/archive/flex-{0}.tar.gz".format(version.dashed)
 
         return url
-
-    def autoreconf(self, spec, prefix):
-        pass
-
-    @when('@:2.6.0')
-    def autoreconf(self, spec, prefix):
-        libtoolize('--install', '--force')
-        autoreconf('--install', '--force')