Bug 1286993 - Add build/pymake to the SEARCH_PATHS in mach_bootstrap. r=gps

It used to be there, but was removed in bug 1151124 because what I
thought was the only use of pymake in mach was removed in that bug.

Which, it turns out, was not the case. So restore it, so that mach
empty-makefiles works again.

--HG--
extra : rebase_source : 15d073a3a95961d75dae168c4ea0ca81d1cd8371

build/mach_bootstrap.py

@@ -51,6 +51,7 @@ SEARCH_PATHS = [
     'python/requests',
     'python/slugid',
     'build',
+    'build/pymake',
     'config',
     'dom/bindings',
     'dom/bindings/parser',