BLD: __SKIMAGE_SETUP__ global variable for detecting build

The __SKIMAGE_SETUP__ global variable is set to True if skimage is being build
from the main setup.py file, False if not.  This behavior is modeled after a
similar setup in scikit-learn

setup.py

@@ -24,6 +24,19 @@ import sys
 import setuptools
 from distutils.command.build_py import build_py
 
+if sys.version_info[0] < 3:
+    import __builtin__ as builtins
+else:
+    import builtins
+
+# This is a bit (!) hackish: we are setting a global variable so that the main
+# skimage __init__ can detect if it is being loaded by the setup routine, to
+# avoid attempting to load components that aren't built yet:
+# the numpy distutils extensions that are used by scikit-image to recursively
+# build the compiled extensions in sub-packages is based on the Python import
+# machinery.
+builtins.__SKIMAGE_SETUP__ = True
+
 
 with open('skimage/__init__.py') as fid:
     for line in fid:

skimage/__init__.py

@@ -56,6 +56,7 @@ img_as_ubyte
 
 """
 
+import sys
 import os.path as osp
 import imp
 import functools
@@ -111,4 +112,19 @@ doctest_verbose.__doc__ = doctest.__doc__
 
 del warnings, functools, osp, imp
 
-from . import __check_build
+try:
+    # This variable is injected in the __builtins__ by the build
+    # process. It used to enable importing subpackages of skimage when
+    # the binaries are not built
+    __SKIMAGE_SETUP__
+except NameError:
+    __SKIMAGE_SETUP__ = False
+
+if __SKIMAGE_SETUP__:
+    sys.stderr.write('Partial import of skimage during the build process.\n')
+    # We are not importing the rest of the scikit during the build
+    # process, as it may not be compiled yet
+else:
+    from . import __check_build
+    from .util.dtype import *
+    __check_build   # avoid flakes unused varaible error