Don't recompile core.a if none of the .o files changed

Before, core.a would be rebuilt on every build, even when none of the
core .o files changed. Now, the timestamps are checked against the
timestamp on core.a first, skipping the build if nothing changed.

Because this uses the current list of .o files, there is a corner case
when a source file is deleted, but no other source file is modified. In
that case, core.a is not rebuilt, even though it should be. However,
this is such a narrow and unrealistic case, that it should pose a real
problem.

This fixes part of arduino#1991

Author: matthijskooijman

app/src/processing/app/debug/Compiler.java

@@ -703,11 +703,39 @@ void compileCore()
     if (variantFolder != null)
       includeFolders.add(variantFolder);
 
+
+    if (variantFolder != null)
+      objectFiles.addAll(compileFiles(buildFolder, variantFolder, true,
+                                      includeFolders));
+
     File afile = new File(buildFolder, "core.a");
 
     List<File> coreObjectFiles = compileFiles(buildFolder, coreFolder, true,
                                               includeFolders);
 
+    // See if the .a file is already uptodate
+    if (afile.exists()) {
+      boolean changed = false;
+      for (File file : coreObjectFiles) {
+        if (file.lastModified() > afile.lastModified()) {
+          changed = true;
+          break;
+        }
+      }
+
+      // If none of the object files is newer than the .a file, don't
+      // bother rebuilding the .a file. There is a small corner case
+      // here: If a source file was removed, but no other source file
+      // was modified, this will not rebuild core.a even when it
+      // should. It's hard to fix and not a realistic case, so it
+      // shouldn't be a problem.
+      if (!changed) {
+        if (verbose)
+          System.out.println(I18n.format(_("Using previously compiled file: {0}"), afile.getPath()));
+        return;
+      }
+    }
+
     // Delete the .a file, to prevent any previous code from lingering
     afile.delete();
 
@@ -732,10 +760,6 @@ void compileCore()
       afile.delete();
       throw e;
     }
-
-    if (variantFolder != null)
-      objectFiles.addAll(compileFiles(buildFolder, variantFolder, true,
-                                      includeFolders));
   }
 
   // 4. link it all together into the .elf file