fix numpy related import error on fresh install

Signed-off-by: masadcv <muhammad.asad@kcl.ac.uk>

Author: masadcv

setup.py

@@ -1,5 +1,4 @@
 import sys
-import numpy
 import setuptools
 from distutils.core import setup
 from distutils.extension import Extension
@@ -10,7 +9,7 @@
 module_name1   = 'maxflow'
 maxflow_source = "maxflow_python/wrap_py{0:}.cpp".format(py_version)
 module1 = Extension(module_name1,
-                    include_dirs = [numpy.get_include(),'./dependency', './maxflow_python'],
+                    include_dirs = ['./dependency', './maxflow_python'],
                     sources = ['maxflow_python/maxflow.cpp', 
                                'maxflow_python/util.cpp',
                                'dependency/maxflow-v3.0/graph.cpp', 
@@ -20,7 +19,7 @@
 module_name2    = 'denseCRF'
 densecrf_source = "densecrf_python/wrap2D_py{0:}.cpp".format(py_version)
 module2 = Extension(module_name2,
-                    include_dirs = [numpy.get_include(), 
+                    include_dirs = [ 
                                     './dependency/densecrf/include', 
                                     './dependency/densecrf/external/liblbfgs/include'], 
                     sources=['./densecrf_python/densecrf.cpp',
@@ -40,7 +39,7 @@
 module_name3    = 'denseCRF3D'
 densecrf_source = "densecrf_python/wrap3D_py{0:}.cpp".format(py_version)
 module3 = Extension(module_name3,
-                    include_dirs = [numpy.get_include(), 
+                    include_dirs = [ 
                                     './dependency/densecrf3d/include', 
                                     './dependency/densecrf/external/liblbfgs/include'], 
                     sources=['./densecrf_python/densecrf3d.cpp',
@@ -69,6 +68,20 @@
     with open('README.md', encoding='utf-8') as f:
         long_description = f.read()
 
+from setuptools.command.build_ext import build_ext
+
+class CustomBuildExtCommand(build_ext):
+    """build_ext command for use when numpy headers are needed."""
+    def run(self):
+
+        # Import numpy here, only when headers are needed
+        import numpy
+
+        # Add numpy headers to include_dirs
+        self.include_dirs.append(numpy.get_include())
+
+        # Call original build_ext command
+        build_ext.run(self)
 
 setup(name=package_name,
       version = "0.1.1",
@@ -87,7 +100,10 @@
             'Programming Language :: Python :: 2',
             'Programming Language :: Python :: 3',
       ],
-      python_requires = '>=3.6')
+      python_requires = '>=3.6',
+      cmdclass = {'build_ext': CustomBuildExtCommand},
+      install_requires=['numpy'],
+    )
 
 
 # to build, run python stup.py build or python setup.py build_ext --inplace