Add ability to disable use of preprocessed output in compilation

This can be done from a buildfile for a scope (including project root scope)
and per target via cc.reprocess:

cc.reprocess = true
obj{hello}: cc.reprocess = false

As as well externally via config.cc.reprocess:

b config.cc.reprocess=true [configure]

1 files changed, 7 insertions, 4 deletions

diff --git a/build2/cc/compile.cxx b/build2/cc/compile.cxx
index 32b5f6e..1eb4c9b 100644
--- a/build2/cc/compile.cxx
+++ b/build2/cc/compile.cxx
@@ -567,11 +567,14 @@ namespace build2
         pair<auto_rmfile, bool> p (inject (act, t, lo, src, dd, u));
         dd.close ();
 
-        // If the preprocessed output is suitable for compilation, pass it
-        // along.
+        // If the preprocessed output is suitable for compilation and is not
+        // disabled, pass it along.
         //
-        if (p.second)
-          md.psrc = move (p.first);
+        if (!p.first.path ().empty () && p.second)
+        {
+          if (!cast_false<bool> (t[c_reprocess]))
+            md.psrc = move (p.first);
+        }
 
         md.mt = u ? timestamp_nonexistent : mt;
       }