Make vcftools actually install its libraries (#4682)

The vcftools package was installing its Perl libraries into a
directory in the staging directory named 'lib', instead of installing
them into `prefix.lib` where they should have been because the value
passed to the configure script was wrong.

The result was that any attempt to run one of the scripts in the
package failed with something like:

> Can't locate VcfStats.pm in @INC (you may need to install the
> VcfStats module) [...]

This fix provides the correct information in `configure_args`.

Tested lightly on CentOS 7.

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

@@ -45,7 +45,7 @@ class Vcftools(AutotoolsPackage):
     # this needs to be in sync with what setup_environment adds to
     # PERL5LIB below
     def configure_args(self):
-        return ['--with-pmdir=lib']
+        return ['--with-pmdir={0}'.format(self.prefix.lib)]
 
     @run_before('install')
     def filter_sbang(self):
@@ -72,4 +72,4 @@ def filter_sbang(self):
             filter_file(match, substitute, *files, **kwargs)
 
     def setup_environment(self, spack_env, run_env):
-        run_env.prepend_path('PERL5LIB', join_path(self.prefix, 'lib'))
+        run_env.prepend_path('PERL5LIB', self.prefix.lib)