[meld] Update build helpers for Python 3.8 compatibility (#322)

[Kai Willadsen <kaiw src gnome org>]

commit 9cb590f9804a89d8914d0d7d6c89c336f6d86d86
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Mon May 6 08:15:23 2019 +1000

    Update build helpers for Python 3.8 compatibility (#322)
    
    The `linux_distribution` helper for the platform module has been removed
    in Python 3.8, so we need an additional helper to check for the Debian-
    style packaging layout.
    
    Really this should be an `install_requires`, but moving our build
    helpers to `setuptools` is a not-insignificant risk that I'd rather not
    take when we're looking at moving to Meson.

 README.md             |  6 ++++++
 meld/build_helpers.py | 29 ++++++++++++++++++++++++-----
 2 files changed, 30 insertions(+), 5 deletions(-)
---
diff --git a/README.md b/README.md
index 2105dbdd..dfb63fb1 100644
--- a/README.md
+++ b/README.md
@@ -33,10 +33,16 @@ And following packages with GObject introspection:
 Build requirements
 ------------------
 
+System packages:
+
 * intltool
 * itstool
 * xmllint
 
+Python packages:
+
+* distro
+
 Building Windows MSIs requires:
 
 * cx_Freeze 5
diff --git a/meld/build_helpers.py b/meld/build_helpers.py
index 5977b3cd..f9fb302b 100644
--- a/meld/build_helpers.py
+++ b/meld/build_helpers.py
@@ -31,6 +31,15 @@ import platform
 import sys
 from distutils.log import info
 
+try:
+    import distro
+except ImportError:
+    python_version = tuple(int(x) for x in platform.python_version_tuple())
+    if python_version >= (3, 8):
+        print(
+            'Missing build requirement "distro" Python module; '
+            'install paths may be incorrect', file=sys.stderr)
+
 
 def has_help(self):
     return "build_help" in self.distribution.cmdclass and os.name != 'nt'
@@ -404,11 +413,21 @@ class install(distutils.command.install.install):
 
     def finalize_options(self):
         special_cases = ('debian', 'ubuntu', 'linuxmint')
-        if (platform.system() == 'Linux' and
-                platform.linux_distribution()[0].lower() in special_cases):
-            # Maintain an explicit install-layout, but use deb by default
-            specified_layout = getattr(self, 'install_layout', None)
-            self.install_layout = specified_layout or 'deb'
+        if platform.system() == 'Linux':
+            # linux_distribution has been removed in Python 3.8; we require
+            # the third-party distro package for future handling.
+            try:
+                distribution = platform.linux_distribution()[0].lower()
+            except AttributeError:
+                try:
+                    distribution = distro.id()
+                except NameError:
+                    distribution = 'unknown'
+
+            if distribution in special_cases:
+                # Maintain an explicit install-layout, but use deb by default
+                specified_layout = getattr(self, 'install_layout', None)
+                self.install_layout = specified_layout or 'deb'
 
         distutils.command.install.install.finalize_options(self)