Add num_jobs as an explicit option so it can be set from other sources

The `-j` flag is only settable via the command line, which makes it hard
to configure when running builds from places like VS, where the flag isn't
easily exposed or configurable.

This lets users configure the number of jobs to be used by default if `-j`
isn't specified, instead of always defaulting to number of cores - 1.

Co-authored-by: Rémi Verschelde <[email protected]>

SConstruct

@@ -234,6 +234,11 @@ opts.Add(BoolVariable("tests", "Build the unit tests", False))
 opts.Add(BoolVariable("fast_unsafe", "Enable unsafe options for faster rebuilds", False))
 opts.Add(BoolVariable("ninja", "Use the ninja backend for faster rebuilds", False))
 opts.Add(BoolVariable("compiledb", "Generate compilation DB (`compile_commands.json`) for external tools", False))
+opts.Add(
+    "num_jobs",
+    "Use up to N jobs when compiling (equivalent to `-j N`). Defaults to max jobs - 1. Ignored if -j is used.",
+    "",
+)
 opts.Add(BoolVariable("verbose", "Enable verbose output for the compilation", False))
 opts.Add(BoolVariable("progress", "Show a progress indicator during compilation", True))
 opts.Add(EnumVariable("warnings", "Level of compilation warnings", "all", ("extra", "all", "moderate", "no")))
@@ -537,16 +542,22 @@ initial_num_jobs = env.GetOption("num_jobs")
 altered_num_jobs = initial_num_jobs + 1
 env.SetOption("num_jobs", altered_num_jobs)
 if env.GetOption("num_jobs") == altered_num_jobs:
-    cpu_count = os.cpu_count()
-    if cpu_count is None:
-        print_warning("Couldn't auto-detect CPU count to configure build parallelism. Specify it with the -j argument.")
+    num_jobs = env.get("num_jobs", "")
+    if str(num_jobs).isdigit() and int(num_jobs) > 0:
+        env.SetOption("num_jobs", num_jobs)
     else:
-        safer_cpu_count = cpu_count if cpu_count <= 4 else cpu_count - 1
-        print(
-            "Auto-detected %d CPU cores available for build parallelism. Using %d cores by default. You can override it with the -j argument."
-            % (cpu_count, safer_cpu_count)
-        )
-        env.SetOption("num_jobs", safer_cpu_count)
+        cpu_count = os.cpu_count()
+        if cpu_count is None:
+            print_warning(
+                "Couldn't auto-detect CPU count to configure build parallelism. Specify it with the `-j` or `num_jobs` arguments."
+            )
+        else:
+            safer_cpu_count = cpu_count if cpu_count <= 4 else cpu_count - 1
+            print(
+                "Auto-detected %d CPU cores available for build parallelism. Using %d cores by default. You can override it with the `-j` or `num_jobs` arguments."
+                % (cpu_count, safer_cpu_count)
+            )
+            env.SetOption("num_jobs", safer_cpu_count)
 
 env.extra_suffix = ""