fix boost build on single-core machines

Summary:
On a single-core Ubuntu VM, getdeps attempts to pass -j0 to Boost Jam,
which fails.

Reviewed By: fanzeyi

Differential Revision: D33675531

fbshipit-source-id: 629ee41448941213aadec10658583d8f984b13c2

build/fbcode_builder/getdeps/buildopts.py

@@ -75,7 +75,7 @@ class BuildOptions(object):
         if not num_jobs:
             import multiprocessing
 
-            num_jobs = multiprocessing.cpu_count() // 2
+            num_jobs = max(1, multiprocessing.cpu_count() // 2)
 
         if not install_dir:
             install_dir = os.path.join(scratch_dir, "installed")