sync-and-gyp: make smarter

This fixes two issues: If gyp is run with the CXX and CC environment
variables set, it will produce different ninja build files.
sync-and-gyp now tracks the values of those variables; if they change,
re-run gyp.

Secondly, we introduced find.py, which is used inside gyp files.  If
the contents of certain directories changes, gyp should be re-run.  So
now we track those directories too.

Review URL: https://codereview.chromium.org/1153083002
diff --git a/bin/sync-and-gyp b/bin/sync-and-gyp
index 825f316..eb0311a 100755
--- a/bin/sync-and-gyp
+++ b/bin/sync-and-gyp
@@ -41,9 +41,12 @@
 
 function gyp_hasher() {
     {
+        echo "$CC"
+        echo "$CXX"
         echo "$GYP_GENERATORS"
         echo "$GYP_DEFINES"
         find gyp -type f -print -exec git hash-object {} \;
+        find bench gm tests -name '*.c*' | LANG= sort
     } | git hash-object --stdin
 }