Fix source release: include mbedTLS source...

And do not automatically `rm -rf` the `nng` and `mbedtls` directories if
they are present.

This will make developing pynng more error-prone--it will be easier to
accidentally build against a stale library--but it is the Right Thing™
and allows source distributions to build.

MANIFEST.in

@@ -2,6 +2,7 @@ include *.sh *.py *.bat 8.txt *.md
 include *.h
 include LICENSE.txt
 recursive-include nng *
+recursive-include mbedtls *
 recursive-include test *.py
 recursive-include pynng *.py
 prune nng/build

pynng/_version.py

@@ -1,3 +1,3 @@
 # This file is imported from __init__.py and exec'd from setup.py
 
-__version__ = "0.6.1"
+__version__ = "0.6.2"

setup.py

@@ -34,12 +34,11 @@ def build_mbedtls(cmake_args):
 
     """
     do = check_call
-    if os.path.exists('mbedtls'):
-        _rmdir('mbedtls')
-    do('git clone --recursive {}'.format(MBEDTLS_REPO), shell=True)
-    # for local hacking, just copy a directory (network connection is slow)
-    # do('cp -r ../mbedtls mbedtls', shell=True)
-    do('git checkout {}'.format(MBEDTLS_REV), shell=True, cwd='mbedtls')
+    if not os.path.exists('mbedtls'):
+        do('git clone --recursive {}'.format(MBEDTLS_REPO), shell=True)
+        # for local hacking, just copy a directory (network connection is slow)
+        # do('cp -r ../mbedtls mbedtls', shell=True)
+        do('git checkout {}'.format(MBEDTLS_REV), shell=True, cwd='mbedtls')
     cwd = 'mbedtls/build'
     os.mkdir(cwd)
     cmake_cmd = ['cmake'] + cmake_args
@@ -64,12 +63,11 @@ def build_nng(cmake_args):
 
     """
     do = check_call
-    if os.path.exists('nng'):
-        _rmdir('nng')
-    do('git clone {}'.format(NNG_REPO), shell=True)
-    # for local hacking, just copy a directory (network connection is slow)
-    # do('cp -r ../nng-clean nng', shell=True)
-    do('git checkout {}'.format(NNG_REV), shell=True, cwd='nng')
+    if not os.path.exists('nng'):
+        do('git clone {}'.format(NNG_REPO), shell=True)
+        # for local hacking, just copy a directory (network connection is slow)
+        # do('cp -r ../nng-clean nng', shell=True)
+        do('git checkout {}'.format(NNG_REV), shell=True, cwd='nng')
     os.mkdir('nng/build')
     cmake_cmd = ['cmake'] + cmake_args
     cmake_cmd += [