setup.py: Build concurrently by default

Because why not? It will be faster - on my machine ~4x faster.

Author: Lorak-mmk

docs/installation.rst

@@ -76,6 +76,9 @@ threads used to build the driver and any C extensions:
     $ # installing from pip
     $ CASS_DRIVER_BUILD_CONCURRENCY=8 pip install scylla-driver
 
+Note that by default (when CASS_DRIVER_BUILD_CONCURRENCY is not specified), concurrency will be equal to the number of
+logical cores on your machine.
+
 OSX Installation Error
 ^^^^^^^^^^^^^^^^^^^^^^
 If you're installing on OSX and have XCode 5.1 installed, you may see an error like this::

setup.py

@@ -206,6 +206,9 @@ def eval_env_var_as_array(varname):
 sys.argv = [a for a in sys.argv if a not in ("--no-murmur3", "--no-libev", "--no-cython", "--no-extensions")]
 
 build_concurrency = int(os.environ.get('CASS_DRIVER_BUILD_CONCURRENCY', '0'))
+if build_concurrency == 0:
+    build_concurrency = None
+
 CASS_DRIVER_BUILD_EXTENSIONS_ARE_MUST = bool(os.environ.get('CASS_DRIVER_BUILD_EXTENSIONS_ARE_MUST', 'no') == 'yes')
 
 class NoPatchExtension(Extension):
@@ -292,7 +295,7 @@ def run(self):
 
 
     def build_extensions(self):
-        if build_concurrency > 1:
+        if build_concurrency is None or build_concurrency > 1:
             self.check_extensions_list(self.extensions)
 
             import multiprocessing.pool
@@ -431,7 +434,6 @@ def run_setup(extensions):
 
     setup(**kw)
 
-
 run_setup(None)
 
 if has_cqlengine: