Force C implementations when using Daala DCT's.

This patch fixes a regression introduced in 1d190950 where the encoder
 was using the 4x4 VP9/AV1 transforms for RDO, but then used the Daala
 transforms for encoding.
The ~2% improvement below comes from forcing the C implementation of the
 4x4 and 8x8 transforms to be used when CONFIG_DAALA_DCT4 and
 CONFIG_DAALA_DCT8 are enabled respectively.

subset-1 (--enable-experimental --enable-daala_dct4):

master@2017-08-21T21:41:18.302Z ->
 master_daala_dct4_use_c@2017-08-22T02:39:14.457Z

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-2.1953 | -1.2044 | -1.1865 |  -1.6173 | -1.7029 | -1.6784 |    -1.7235

Change-Id: I44d2b24094e89b2857ae03d743180e706cef45eb