Add ability to serialize compilation/linking in cc rules

Specifically, both the C/C++ compiler and link rules now recognize the
cc.serialize boolean variable which instructs them to compiler/link
serially with regards to any other recipe.

This is primarily useful when compiling large translation units or linking
large binaries that require so much memory that doing that in parallel with
other compilation/linking jobs is likely to summon the OOM killer. For
example:

obj{memory-hog}: cc.serialize = true

1 files changed, 10 insertions, 0 deletions

diff --git a/libbuild2/cc/compile-rule.cxx b/libbuild2/cc/compile-rule.cxx
index d490c8e..2e4775e 100644
--- a/libbuild2/cc/compile-rule.cxx
+++ b/libbuild2/cc/compile-rule.cxx
@@ -7830,6 +7830,14 @@ namespace build2
       if (!env.empty ())
         env.push_back (nullptr);
 
+      // We have no choice but to serialize early if we want the command line
+      // printed shortly before actually executing the compiler. Failed that,
+      // it may look like we are still executing in parallel.
+      //
+      scheduler::alloc_guard jobs_ag;
+      if (!ctx.dry_run && cast_false<bool> (t[c_serialize]))
+        jobs_ag = scheduler::alloc_guard (*ctx.sched, phase_unlock (nullptr));
+
       // With verbosity level 2 print the command line as if we are compiling
       // the source file, not its preprocessed version (so that it's easy to
       // copy and re-run, etc). Only at level 3 and above print the real deal.
@@ -7994,6 +8002,8 @@ namespace build2
           throw failed ();
         }
 
+        jobs_ag.deallocate ();
+
         if (md.deferred_failure)
           fail << "expected error exit status from " << x_lang << " compiler";
       }