--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -26,7 +26,7 @@ from .. import compilers
 from ..compilers import CompilerArgs
 from ..linkers import ArLinker
 from ..mesonlib import File, MesonException, OrderedSet
-from ..mesonlib import get_compiler_for_source
+from ..mesonlib import get_compiler_for_source, commonpath
 from .backends import CleanTrees, InstallData
 from ..build import InvalidArguments
 
@@ -1166,8 +1166,8 @@ int dummy;
                 # Check if the vala file is in a subdir of --basedir
                 abs_srcbasedir = os.path.join(self.environment.get_source_dir(), target.get_subdir())
                 abs_vala_file = os.path.join(self.environment.get_build_dir(), vala_file)
-                if PurePath(os.path.commonpath((abs_srcbasedir, abs_vala_file))) == PurePath(abs_srcbasedir):
-                    vala_c_subdir = PurePath(abs_vala_file).parent.relative_to(abs_srcbasedir)
+                if PurePath(commonpath((abs_srcbasedir, abs_vala_file))) == PurePath(abs_srcbasedir):
+                    vala_c_subdir = str(PurePath(abs_vala_file).parent.relative_to(abs_srcbasedir))
                     vala_c_file = os.path.join(vala_c_subdir, vala_c_file)
             else:
                 path_to_target = os.path.join(self.build_to_src, target.get_subdir())
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -17,7 +17,7 @@ import pickle, os, uuid
 import sys
 from pathlib import PurePath
 from collections import OrderedDict
-from .mesonlib import MesonException
+from .mesonlib import MesonException, commonpath
 from .mesonlib import default_libdir, default_libexecdir, default_prefix
 import ast
 
@@ -286,7 +286,7 @@ class CoreData:
             # commonpath will always return a path in the native format, so we
             # must use pathlib.PurePath to do the same conversion before
             # comparing.
-            if os.path.commonpath([value, prefix]) != str(PurePath(prefix)):
+            if commonpath([value, prefix]) != str(PurePath(prefix)):
                 m = 'The value of the {!r} option is {!r} which must be a ' \
                     'subdir of the prefix {!r}.\nNote that if you pass a ' \
                     'relative path, it is assumed to be a subdir of prefix.'
--- a/mesonbuild/mesonlib.py
+++ b/mesonbuild/mesonlib.py
@@ -935,6 +935,30 @@ def detect_subprojects(spdir_name, curre
                 result[basename] = [trial]
     return result
 
+def commonpath(paths):
+    '''
+    For use on Python 3.4 where os.path.commonpath is not available.
+    '''
+    if sys.version_info >= (3, 5):
+        return os.path.commonpath(paths)
+
+    import pathlib
+    if not paths:
+        raise ValueError('commonpath() arg is an empty sequence')
+    common = pathlib.PurePath(paths[0])
+    for path in paths[1:]:
+        new = []
+        path = pathlib.PurePath(path)
+        for c, p in zip(common.parts, path.parts):
+            if c != p:
+                break
+            new.append(c)
+        if not new:
+            raise ValueError("Can't mix absolute and relative paths") from None
+        new = os.path.join(*new)
+        common = pathlib.PurePath(new)
+    return str(common)
+
 class OrderedSet(collections.MutableSet):
     """A set that preserves the order in which items are added, by first
     insertion.
--- a/mesonbuild/mesonmain.py
+++ b/mesonbuild/mesonmain.py
@@ -274,8 +274,8 @@ def run_script_command(args):
     return cmdfunc(cmdargs)
 
 def run(original_args, mainfile=None):
-    if sys.version_info < (3, 5):
-        print('Meson works correctly only with python 3.5+.')
+    if sys.version_info < (3, 4):
+        print('Meson works correctly only with python 3.4+.')
         print('You have python %s.' % sys.version)
         print('Please update your environment')
         return 1
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -427,6 +427,24 @@ class InternalTests(unittest.TestCase):
         kwargs = {'sources': [1, 2, 3], 'pch_sources': [4, 5, 6]}
         self.assertEqual([[1, 2, 3], [4, 5, 6]], extract(kwargs, 'sources', 'pch_sources'))
 
+    def test_commonpath(self):
+        from os.path import sep
+        commonpath = mesonbuild.mesonlib.commonpath
+        self.assertRaises(ValueError, commonpath, [])
+        self.assertEqual(commonpath(['/usr', '/usr']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr', '/usr/']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr', '/usr/bin']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr/', '/usr/bin']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr/./', '/usr/bin']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr/bin', '/usr/bin']), sep + 'usr' + sep + 'bin')
+        self.assertEqual(commonpath(['/usr//bin', '/usr/bin']), sep + 'usr' + sep + 'bin')
+        self.assertEqual(commonpath(['/usr/./bin', '/usr/bin']), sep + 'usr' + sep + 'bin')
+        self.assertEqual(commonpath(['/usr/local', '/usr/lib']), sep + 'usr')
+        self.assertEqual(commonpath(['/usr', '/bin']), sep)
+        prefix = '/some/path/to/prefix'
+        libdir = '/some/path/to/prefix/libdir'
+        self.assertEqual(commonpath([prefix, libdir]), str(PurePath(prefix)))
+
 
 class BasePlatformTests(unittest.TestCase):
     def setUp(self):