Max-jobs is now defined on a platform basis

This is due to Darwin (MacOS) having a different Max-Jobs limit
_platform/platform.py: Adding max-jobs call

_project.py: redirected 'max-jobs' to point at the platform

buildstream/_platform/platform.py

@@ -68,6 +68,9 @@ class Platform():
             raise PlatformError("Platform needs to be initialized first")
         return cls._instance
 
+    def get_cpu_count(self, cap=None):
+        return min(len(os.sched_getaffinity(0)), cap)
+
     ##################################################################
     #                       Platform properties                      #
     ##################################################################

buildstream/_project.py

@@ -38,6 +38,7 @@ from ._loader import Loader
 from .element import Element
 from ._message import Message, MessageType
 from ._includes import Includes
+from ._platform import Platform
 
 
 # Project Configuration file
@@ -617,7 +618,8 @@ class Project():
         # Based on some testing (mainly on AWS), maximum effective
         # max-jobs value seems to be around 8-10 if we have enough cores
         # users should set values based on workload and build infrastructure
-        output.base_variables['max-jobs'] = str(min(len(os.sched_getaffinity(0)), 8))
+        platform = Platform.get_platform()
+        output.base_variables['max-jobs'] = str(platform.get_cpu_count(8))
 
         # Export options into variables, if that was requested
         output.options.export_variables(output.base_variables)