diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index cae40095e..7b4059e51 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -13,7 +13,8 @@
 # limitations under the License.
 
 import pickle, os, uuid
-from .mesonlib import MesonException, default_libdir, default_libexecdir, default_prefix
+from .mesonlib import MesonException, commonpath
+from .mesonlib import default_libdir, default_libexecdir, default_prefix
 
 version = '0.38.1'
 backendlist = ['ninja', 'vs2010', 'vs2015', 'xcode']
@@ -158,7 +159,7 @@ class CoreData:
         if option.endswith('dir') and os.path.isabs(value) and \
            option not in builtin_dir_noprefix_options:
             # Value must be a subdir of the prefix
-            if os.path.commonpath([value, prefix]) != prefix:
+            if commonpath([value, prefix]) != 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.'
@@ -194,9 +195,9 @@ class CoreData:
         elif optname in self.builtins:
             prefix = self.builtins['prefix'].value
             value = self.sanitize_dir_option_value(prefix, optname, value)
-            self.builtins[optname].set_value(value)
         else:
             raise RuntimeError('Tried to set unknown builtin option %s.' % optname)
+        self.builtins[optname].set_value(value)
 
 def load(filename):
     load_fail_msg = 'Coredata file {!r} is corrupted. Try with a fresh build tree.'.format(filename)
diff --git a/mesonbuild/mesonlib.py b/mesonbuild/mesonlib.py
index 2ad43c823..f0b20e1bd 100644
--- a/mesonbuild/mesonlib.py
+++ b/mesonbuild/mesonlib.py
@@ -496,3 +496,28 @@ def Popen_safe(args, write=None, stderr=subprocess.PIPE, **kwargs):
                          stderr=stderr, **kwargs)
     o, e = p.communicate(write)
     return p, o, e
+
+def commonpath(paths):
+    '''
+    For use on Python 3.4 where os.path.commonpath is not available.
+    We currently use it everywhere so this receives enough testing.
+    '''
+    # XXX: Replace me with os.path.commonpath when we start requiring Python 3.5
+    import pathlib
+    if not paths:
+        raise ValueError('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)
+        # Don't convert '' into '.'
+        if not new:
+            common = ''
+            break
+        new = os.path.join(*new)
+        common = pathlib.PurePath(new)
+    return str(common)
diff --git a/run_tests.py b/run_tests.py
index 2dfbaffad..005717e07 100755
--- a/run_tests.py
+++ b/run_tests.py
@@ -20,9 +20,11 @@ from mesonbuild import mesonlib
 
 if __name__ == '__main__':
     returncode = 0
+    print('Running unittests.\n')
     if mesonlib.is_linux():
-        print('Running unittests.\n')
         returncode += subprocess.call([sys.executable, 'run_unittests.py', '-v'])
+    else:
+        returncode += subprocess.call([sys.executable, 'run_unittests.py', '-v', 'InternalTests'])
     # Ubuntu packages do not have a binary without -6 suffix.
     if shutil.which('arm-linux-gnueabihf-gcc-6') and not platform.machine().startswith('arm'):
         print('Running cross compilation tests.\n')
diff --git a/run_unittests.py b/run_unittests.py
index a1d99f392..e8659f40a 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -153,6 +153,23 @@ class InternalTests(unittest.TestCase):
         a = ['-Ldir', '-Lbah'] + a
         self.assertEqual(a, ['-Ibar', '-Ifoo', '-Ibaz', '-I..', '-I.', '-Ldir', '-Lbah', '-Werror', '-O3', '-O2', '-Wall'])
 
+    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)
+        self.assertEqual(commonpath(['/usr', 'bin']), '')
+        self.assertEqual(commonpath(['blam', 'bin']), '')
+
 
 class LinuxlikeTests(unittest.TestCase):
     def setUp(self):