goma.mk: do not start goma compiler_proxy if NOSTART_GOMA is set.

During the development of goma client, we sometimes need to use goma
compiler_proxy, which has already been running.

Test: NOSTART_GOMA=1 USE_GOMA=1 m.

Change-Id: Ifa969c40871325fd43ee6443ddbe3b18fde62c5a
diff --git a/core/goma.mk b/core/goma.mk
index 52726be..4e8318a 100644
--- a/core/goma.mk
+++ b/core/goma.mk
@@ -49,7 +49,9 @@
   # gomacc can start goma client's daemon process automatically, but
   # it is safer and faster to start up it beforehand. We run this as a
   # background process so this won't slow down the build.
-  $(shell ( $(goma_ctl) ensure_start ) &> /dev/null &)
+  ifndef NOSTART_GOMA
+    $(shell ( $(goma_ctl) ensure_start ) &> /dev/null &)
+  endif
 
   goma_ctl :=
   goma_dir :=