cantera: Fix passing 'python_prefix' variable into installation path

The '${python_prefix}' substring for installation prefix path
was accepted as mapping key for '.format()' function resulting in
a 'KeyError' failure of 'cantera/interfaces/cython/SConscript' script
in case of `env[libdirname] == 'lib64'`.

Moreover the early applied pull request[1] didn't take into account
the additional setting of installation prefix path in the cases
when 'libdirname' takes values different from 'lib64'.

This patch resolves both those issues.

[1]: https://github.com/Cantera/cantera/pull/661

interfaces/cython/SConscript

@@ -101,11 +101,11 @@ elif localenv['python_prefix']:
         extra = ''
     elif localenv['OS'] == 'Darwin':
         extra = localenv.subst(' --prefix=${python_prefix}')
-    elif localenv['libdirname'] == 'lib64':
-        # 64-bit RHEL / Fedora
+    elif localenv['libdirname'] != 'lib':
+        # 64-bit RHEL / Fedora etc. or e.g. x32 Gentoo profile
         extra = localenv.subst(
             ' --prefix=${python_prefix}'
-            ' --install-lib=${python_prefix}/lib64/python{}/site-packages'.format(py_version))
+            ' --install-lib=${{python_prefix}}/${{libdirname}}/python{}/site-packages'.format(py_version))
     else:
         extra = '--user'
         localenv.AppendENVPath(