Make sure to use defined(__GNUC__) instead of bare __GNUC__ in flann2 header.

TheGoddessInari · alalek · commit 960bb540ecdd · 2019-08-05T17:19:12.000+03:00
This fixes a compiler error on MSVC with Unreal Engine (at least). backport of commit: 4e998c9
diff --git a/modules/flann/include/opencv2/flann/dist.h b/modules/flann/include/opencv2/flann/dist.h
@@ -441,7 +441,7 @@ struct Hamming
             result = vgetq_lane_s32 (vreinterpretq_s32_u64(bitSet2),0);
             result += vgetq_lane_s32 (vreinterpretq_s32_u64(bitSet2),2);
         }
-#elif __GNUC__
+#elif defined(__GNUC__)
         {
             //for portability just use unsigned long -- and use the __builtin_popcountll (see docs for __builtin_popcountll)
             typedef unsigned long long pop_t;

Original file line number	Diff line number	Diff line change
`@@ -441,7 +441,7 @@ struct Hamming`
`441`	`441`	`result = vgetq_lane_s32 (vreinterpretq_s32_u64(bitSet2),0);`
`442`	`442`	`result += vgetq_lane_s32 (vreinterpretq_s32_u64(bitSet2),2);`
`443`	`443`	`}`
`444`		`-#elif __GNUC__`
	`444`	`+#elif defined(__GNUC__)`
`445`	`445`	`{`
`446`	`446`	`//for portability just use unsigned long -- and use the __builtin_popcountll (see docs for __builtin_popcountll)`
`447`	`447`	`typedef unsigned long long pop_t;`