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Guard _BitScanReverse on MSVC so that MSVC 6 doesn't break.

This also adds some extra casts to shut up compiler warnings
 reported on MSVC 6 where there is implicit truncation for the
 arguments of bitexact_cos().
Lacking access to CLZ/BSR will make the code a fair bit slower but
 that is better than failing to compile.

--- a/celt/bands.c

+++ b/celt/bands.c

@@ -923,8 +923,8 @@

          fill &= ((1<<B)-1)<<B;

          delta = 16384;

       } else {

-         imid = bitexact_cos(itheta);

-         iside = bitexact_cos(16384-itheta);

+         imid = bitexact_cos((opus_int16)itheta);

+         iside = bitexact_cos((opus_int16)(16384-itheta));

          /* This is the mid vs side allocation that minimizes squared error

             in that band. */

          delta = FRAC_MUL16((N-1)<<7,bitexact_log2tan(iside,imid));

--- a/celt/ecintrin.h

+++ b/celt/ecintrin.h

@@ -48,7 +48,7 @@

 /*Count leading zeros.

   This macro should only be used for implementing ec_ilog(), if it is defined.

   All other code should use EC_ILOG() instead.*/

-#if defined(_MSC_VER)

+#if defined(_MSC_VER) && (_MSC_VER >= 1400)

 # include <intrin.h>

 /*In _DEBUG mode this is not an intrinsic by default.*/

 # pragma intrinsic(_BitScanReverse)

--- a/celt/entcode.c

+++ b/celt/entcode.c

@@ -33,6 +33,11 @@

 #include "arch.h"

 #if !defined(EC_CLZ)

+/*This is a fallback for systems where we don't know how to access

+   a BSR or CLZ instruction (see ecintrin.h).

+  If you are optimizing Opus on a new platform and it has a native CLZ or

+   BZR (e.g. cell, MIPS, x86, etc) then making it available to Opus will be

+   an easy performance win.*/

 int ec_ilog(opus_uint32 _v){

   /*On a Pentium M, this branchless version tested as the fastest on

      1,000,000,000 random 32-bit integers, edging out a similar version with