[SCons] Fix default Python module installation path on Debian/Ubuntu

If Cantera is being installed to /usr/local, then on Debian-based distros, we
want the Python module to end up in /usr/local/lib/pythonX.Y/dist-packages,
which is actually the default behavior. However, /usr/local/... is *not* the
default installation prefix for Python modules on some other distros (On Fedora,
at least, it's /usr/) so using the default needs to be conditioned on the actual
distro.

SConstruct

@@ -1150,6 +1150,10 @@ if any(name.startswith('/usr/lib64/python') for name in sys.path):
 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)
+
 # Directories where things will be after actually being installed. These
 # variables are the ones that are used to populate header files, scripts, etc.
 env['ct_installroot'] = env['prefix']

interfaces/cython/SConscript

@@ -81,7 +81,10 @@ def install_module(prefix, python_version):
             extra += ' --prefix=""'
     elif prefix:
         # A specific location for the Cantera python module has been given
-        if localenv['OS'] == 'Darwin':
+        if localenv['debian'] and localenv.subst(prefix) == '/usr/local':
+            # Installation to /usr/local is the default on Debian-based distributions
+            extra = ''
+        elif localenv['OS'] == 'Darwin':
             extra = localenv.subst(' --prefix=${python%s_prefix}' % ver)
         elif localenv['libdirname'] == 'lib64':
             # 64-bit RHEL / Fedora