exec_command.py states in comments it uses /bin/sh on POSIX for subprocess but incorrectly uses shell from environment. This makes compiles fail when compilation is done using the account 'nobody' which is default on pkg-fallout runs.

PR:		ports/225156
Submitted by:	self
Reported by:	self
Reviewed by:	antoine
Approved by: 	antoine

math/py-numpy/files/patch-numpy-distutils-exec_command.py

@@ -0,0 +1,11 @@
+--- numpy/distutils/exec_command.py.orig	2017-09-29 20:10:10 UTC
++++ numpy/distutils/exec_command.py
+@@ -231,7 +231,7 @@ def _exec_command(command, use_shell=Non
+ 
+     if os.name == 'posix' and use_shell:
+         # On POSIX, subprocess always uses /bin/sh, override
+-        sh = os.environ.get('SHELL', '/bin/sh')
++        sh = '/bin/sh'
+         if is_sequence(command):
+             command = [sh, '-c', ' '.join(command)]
+         else: