mac builder broken due to perl

See

http://build.chromium.org/p/client.nacl.ports/builders/

In patching configure, mac builders get a 'Permission denied' error. Happens 
with mac-clang, mac-newlib, mac-clang, mac-pnacl + similar nightly mac builders.

Relevant failures -

######################################################################
Patching configure
######################################################################
Failed to find patch 0 location in configure script: configure
Failed to find patch 1 location in configure script: configure
Failed to find patch 2 location in configure script: configure
Traceback (most recent call last):
  File "/Volumes/data/b/build/slave/naclports/build/src/build_tools/patch_configure.py", line 108, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/Volumes/data/b/build/slave/naclports/build/src/build_tools/patch_configure.py", line 101, in main
    with open(options.configure, 'w') as output_file:
IOError: [Errno 13] Permission denied: 'configure'
naclports: Building perl: failed.
naclports: Build FAILED for perl (i686/clang-newlib)
@@@STEP_FAILURE@@@

Added BUILD_OS=linux for now (as suggested by sbc), but I guess a better fix is 
needed.

Original issue reported on code.google.com by gauravas...@gmail.com on 11 Jun 2015 at 6:00

bpd1069 / naclports

mac builder broken due to perl #220