scons: Use parallel builds by default.

scons/gallium.py

@@ -163,6 +163,25 @@ def createInstallMethods(env):
     env.AddMethod(install_shared_library, 'InstallSharedLibrary')
 
 
+def num_jobs():
+    try:
+        return int(os.environ['NUMBER_OF_PROCESSORS'])
+    except (ValueError, KeyError):
+        pass
+
+    try:
+        return os.sysconf('SC_NPROCESSORS_ONLN')
+    except (ValueError, OSError, AttributeError):
+        pass
+
+    try:
+        return int(os.popen2("sysctl -n hw.ncpu")[1].read())
+    except ValueError:
+        pass
+
+    return 1
+
+
 def generate(env):
     """Common environment generation code"""
 
@@ -207,6 +226,10 @@ def generate(env):
     env.SConsignFile(os.path.join(build_dir, '.sconsign'))
     env.CacheDir('build/cache')
 
+    # Parallel build
+    if env.GetOption('num_jobs') <= 1:
+        env.SetOption('num_jobs', num_jobs())
+
     # C preprocessor options
     cppdefines = []
     if debug: