cantera: Sconstruct 'libdirname' env PathVariable

Some distributions (e.g. Fedora/RHEL) use 'lib64' instead of 'lib'
on 64-bit systems or could use some other library directory name
instead of 'lib' depends on architecture and profile
(e.g. Gentoo 'libx32' on x32 profile).
If user didn't set 'libdirname' configuration variable then
set it to default value 'lib'

This commit is related to early closed issue:
#318

SConstruct

@@ -334,6 +334,14 @@ config_options = [
         'prefix',
         'Set this to the directory where Cantera should be installed.',
         defaults.prefix, PathVariable.PathAccept),
+    PathVariable(
+        'libdirname',
+        """Set this to the directory where Cantera libraries should be installed.
+           Some distributions (e.g. Fedora/RHEL) use 'lib64' instead of 'lib' on 64-bit systems
+           or could use some other library directory name instead of 'lib' depends
+           on architecture and profile (e.g. Gentoo 'libx32' on x32 profile).
+           If user didn't set 'libdirname' configuration variable set it to default value 'lib'""",
+        'lib', PathVariable.PathAccept),
     EnumVariable(
         'python_package',
         """If you plan to work in Python, then you need the ``full`` Cantera Python
@@ -1346,12 +1354,6 @@ if env['matlab_toolbox'] == 'y':
 # *** Set additional configuration variables ***
 # **********************************************
 
-# Some distributions (e.g. Fedora/RHEL) use 'lib64' instead of 'lib' on 64-bit systems
-if any(name.startswith('/usr/lib64/python') for name in sys.path):
-    env['libdirname'] = 'lib64'
-else:
-    env['libdirname'] = 'lib'
-
 # On Debian-based systems, need to special-case installation to
 # /usr/local because of dist-packages vs site-packages
 env['debian'] = any(name.endswith('dist-packages') for name in sys.path)