Re: Replace uses of deprecated Python module distutils.sysconfig

I wrote:
> Anyway, based on these results, we might have better luck switching to
> sysconfig after we start forcing python3.

On the other hand, that answer is not back-patchable, and we surely
need a back-patchable fix, because people will try to build the
back branches against newer pythons.

Based on the buildfarm results so far, the problem can be described
as "some installations say /usr/local when they should have said /usr".
I experimented with the attached delta patch and it fixes the problem
on my Debian 9 image.  (I don't know Python, so there may be a better
way to do this.)  We'd have to also bump the minimum 3.x version to
3.2, but that seems very unlikely to bother anyone.

            regards, tom lane

diff --git a/config/python.m4 b/config/python.m4
index 8ca1eaa64b..c65356c6ac 100644
--- a/config/python.m4
+++ b/config/python.m4
@@ -56,13 +56,20 @@ AC_MSG_RESULT([$python_configdir])

 AC_MSG_CHECKING([Python include directories])
 python_includespec=`${PYTHON} -c "
-import sysconfig
-a = '-I' + sysconfig.get_path('include')
-b = '-I' + sysconfig.get_path('platinclude')
+import sysconfig, os.path
+a = sysconfig.get_path('include')
+b = sysconfig.get_path('platinclude')
+# Some versions of sysconfig report '/usr/local/include'
+# when they should have said '/usr/include'
+if not os.path.exists(a + '/Python.h'):
+    aalt = a.replace('/usr/local/', '/usr/', 1)
+    if os.path.exists(aalt + '/Python.h'):
+        a = aalt
+        b = b.replace('/usr/local/', '/usr/', 1)
 if a == b:
-    print(a)
+    print('-I' + a)
 else:
-    print(a + ' ' + b)"`
+    print('-I' + a + ' -I' + b)"`
 if test "$PORTNAME" = win32 ; then
     python_includespec=`echo $python_includespec | sed 's,[[\]],/,g'`
 fi
diff --git a/configure b/configure
index 9c856cb1d5..f88db9467d 100755
--- a/configure
+++ b/configure
@@ -10370,13 +10370,20 @@ $as_echo "$python_configdir" >&6; }
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking Python include directories" >&5
 $as_echo_n "checking Python include directories... " >&6; }
 python_includespec=`${PYTHON} -c "
-import sysconfig
-a = '-I' + sysconfig.get_path('include')
-b = '-I' + sysconfig.get_path('platinclude')
+import sysconfig, os.path
+a = sysconfig.get_path('include')
+b = sysconfig.get_path('platinclude')
+# Some versions of sysconfig report '/usr/local/include'
+# when they should have said '/usr/include'
+if not os.path.exists(a + '/Python.h'):
+    aalt = a.replace('/usr/local/', '/usr/', 1)
+    if os.path.exists(aalt + '/Python.h'):
+        a = aalt
+        b = b.replace('/usr/local/', '/usr/', 1)
 if a == b:
-    print(a)
+    print('-I' + a)
 else:
-    print(a + ' ' + b)"`
+    print('-I' + a + ' -I' + b)"`
 if test "$PORTNAME" = win32 ; then
     python_includespec=`echo $python_includespec | sed 's,[\],/,g'`
 fi