Fix get_include for nonstandard installations

This fixes pybind#1425.

pybind11/__init__.py

@@ -1,28 +1,12 @@
 from ._version import version_info, __version__  # noqa: F401 imported but unused
 
 
-def get_include(user=False):
-    from distutils.dist import Distribution
-    import os
+def get_include():
+    from os import pardir
+    from os.path import abspath, dirname, join
+    from sysconfig import get_config_var
     import sys
-
-    # Are we running in a virtual environment?
-    virtualenv = hasattr(sys, 'real_prefix') or \
-        sys.prefix != getattr(sys, "base_prefix", sys.prefix)
-
-    if virtualenv:
-        return os.path.join(sys.prefix, 'include', 'site',
-                            'python' + sys.version[:3])
-    else:
-        dist = Distribution({'name': 'pybind11'})
-        dist.parse_config_files()
-
-        dist_cobj = dist.get_command_obj('install', create=True)
-
-        # Search for packages in user's home directory?
-        if user:
-            dist_cobj.user = user
-            dist_cobj.prefix = ""
-        dist_cobj.finalize_options()
-
-        return os.path.dirname(dist_cobj.install_headers)
+    prefix = abspath(join(dirname(__file__), pardir, pardir, pardir, pardir))
+    relative_include = get_config_var('INCLUDEPY').replace(
+        get_config_var('prefix'), '')
+    return join(prefix, relative_include)

pybind11/__main__.py

@@ -10,8 +10,7 @@
 def print_includes():
     dirs = [sysconfig.get_path('include'),
             sysconfig.get_path('platinclude'),
-            get_include(),
-            get_include(True)]
+            get_include()]
 
     # Make unique but preserve order
     unique_dirs = []