Fix for nghttp2 installation (#5904)

nghttp2 depends on python, and requires the install directory to exist,
and contain the python module directory which it will eventually
install the python modules into.

var/spack/repos/builtin/packages/nghttp2/package.py

@@ -33,3 +33,22 @@ class Nghttp2(AutotoolsPackage):
     url      = "https://github.com/nghttp2/nghttp2/releases/download/v1.26.0/nghttp2-1.26.0.tar.gz"
 
     version('1.26.0', '83fa813b22bacbc6ea80dfb24847569f')
+
+    depends_on('python@2.7:', type=('build', 'run'))
+    depends_on('py-cython@0.19:', type=('build', 'run'))
+    depends_on('py-setuptools', type=('build'))
+
+    def setup_environment(self, spack_env, run_env):
+        site_packages_dir = '/'.join(
+            [self.spec.prefix.lib,
+             ('python' + str(self.spec['python'].version.up_to(2))),
+             'site-packages'])
+        spack_env.prepend_path('PYTHONPATH', site_packages_dir)
+
+    @run_before('install')
+    def ensure_install_dir_exists(self):
+        site_packages_dir = '/'.join(
+            [self.spec.prefix.lib,
+             ('python' + str(self.spec['python'].version.up_to(2))),
+             'site-packages'])
+        mkdirp(site_packages_dir)