Improves gevent recipe robustness

Tries to match and replace patterns to be moved from one environment
variable to another rather than hard coding it.
That way unexpected flags added to parent recipes hierarchy also get
handled.
This issue popped when testing 
kivy#793

Author: AndreMiras

pythonforandroid/recipes/gevent/__init__.py

@@ -26,6 +26,8 @@ def get_recipe_env(self, arch=None, with_flags_in_cc=True):
         env['LIBS'] = ''.join(re.findall(regex, env['LDFLAGS'])).strip()
         env['LDFLAGS'] = re.sub(regex, '', env['LDFLAGS'])
         info('Moved "{}" from LDFLAGS to LIBS.'.format(env['LIBS']))
+        # linker to use the correct gcc (cross compiler) plus additional libs
+        env['LDSHARED'] = '{} -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions {}'.format(env['CC'], env['LIBS'])
         return env