Fix mingw build error

Fixes an issue where newer versions of GCC would try to compile the
visual studio inline assembly.

Fixes issue #571

Change-Id: I8e4500a9277447fdab0b55e5efa1f24ca97bdc03

third_party/libyuv/source/scale.c

diff --git a/third_party/libyuv/source/scale.c b/third_party/libyuv/source/scale.c
index c142a17..72a817d 100644
--- a/third_party/libyuv/source/scale.c
+++ b/third_party/libyuv/source/scale.c
@@ -632,7 +632,7 @@
   { 65536 / 3, 65536 / 3, 65536 / 2, 65536 / 3, 65536 / 3, 65536 / 2, 0, 0 };
 #endif
 
-#if defined(_M_IX86) && !defined(YUV_DISABLE_ASM)
+#if defined(_M_IX86) && !defined(YUV_DISABLE_ASM) && defined(_MSC_VER)
 
 #define HAS_SCALEROWDOWN2_SSE2
 // Reads 32 pixels, throws half away and writes 16 pixels.