AU: locate bsdiff/bspatch in $PATH, rather than hard coding their location.

BUG=8484
TEST=overloaded bs{diff,patch} with a shell script wrapper, put it
early in $PATH, ran unittests, and verified my wrapper was called.

Change-Id: I16c6a9cea6d7268f7a0ae423fc04c6025b18f0df

Review URL: http://codereview.chromium.org/4114006
diff --git a/subprocess.cc b/subprocess.cc
index 49fc4c3..af5fb75 100755
--- a/subprocess.cc
+++ b/subprocess.cc
@@ -162,7 +162,8 @@
       NULL,  // working directory
       argv.get(),
       argp,
-      static_cast<GSpawnFlags>(G_SPAWN_STDERR_TO_DEV_NULL | flags),  // flags
+      static_cast<GSpawnFlags>(G_SPAWN_STDERR_TO_DEV_NULL |
+                               G_SPAWN_SEARCH_PATH | flags),  // flags
       GRedirectStderrToStdout,  // child setup function
       NULL,  // data for child setup function
       &child_stdout,