Build fails on optionrom

Git pseudo-bisected (focused on optionrom commits) it to this commit.

commit cdbd727c20ad7aac7797dc8c95e485e1a4c6901b
Author: Richard Henderson <email address hidden>
Date:   Thu Jul 7 21:49:36 2016 -0700

    build: Use $(AS) for optionrom explicitly


Build output (non-verbose):

  AS    optionrom/linuxboot.o
cpp: fatal error: '-c' is not a valid option to the preprocessor
compilation terminated.
cpp: fatal error: '-c' is not a valid option to the preprocessor
compilation terminated.
  CC    optionrom/linuxboot_dma.o
  CC    /home/bkamath/dev/workspace/block-2/mothra/output/sp0/targetqga/main.o
  AS    optionrom/kvmvapic.o
cpp: fatal error: '-c' is not a valid option to the preprocessor
compilation terminated.

Steps to reproduce:
Using buildroot and overriding qemu version to 2.7.0
Fedora 24, cpp (GCC) 6.2.1 20160916 (Red Hat 6.2.1-2)

I tried first just building without the -c option but it hangs indefinitely. Reverting the above listed commit fixes the problem on my platform. I didn't dive much further into this, because this seems like a regression.