Add a C fallback for get_msb() and change inline to INLINE.

For systems without __builtin_clz() or _BitScanReverse(), taken from libwep

Change-Id: Iead257efc1772c466c79e1dc0356ed571d38d43e

vp9/common/vp9_systemdependent.h

@@ -12,7 +12,6 @@
 #define VP9_COMMON_VP9_SYSTEMDEPENDENT_H_
 
 #ifdef _MSC_VER
-#include <intrin.h>
 #include <math.h>
 #define snprintf _snprintf
 #endif
@@ -35,15 +34,40 @@ static int round(double x) {
 }
 #endif
 
-static const inline int get_msb(int x) {
-#ifdef _MSC_VER
-int r = 0;
-_BitScanReverse(&r, x);
-return r;
-#else
-return 31 ^ __builtin_clz(x);
-#endif
+// use GNU builtins where available.
+#if defined(__GNUC__) && \
+    ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
+static INLINE int get_msb(unsigned int n) {
+  return 31 ^ __builtin_clz(n);
 }
+#elif defined(_MSC_VER) && _MSC_VER > 1310 && \
+      (defined(_M_X64) || defined(_M_IX86))
+#include <intrin.h>
+#pragma intrinsic(_BitScanReverse)
+
+static INLINE int get_msb(unsigned int n) {
+  unsigned long first_set_bit;
+  _BitScanReverse(&first_set_bit, n);
+  return first_set_bit;
+}
+#else
+// Returns (int)floor(log2(n)). n must be > 0.
+static INLINE int get_msb(unsigned int n) {
+  int log = 0;
+  unsigned int value = n;
+  int i;
+
+  for (i = 4; i >= 0; --i) {
+    const int shift = (1 << i);
+    const unsigned int x = value >> shift;
+    if (x != 0) {
+      value = x;
+      log += shift;
+    }
+  }
+  return log;
+}
+#endif
 
 struct VP9Common;
 void vp9_machine_specific_config(struct VP9Common *cm);