Update unpack_template.h (pandas-dev#14441)

USE_CASE_RANGE is a GNU C feature. This change will activate
USE_CASE_RANGE on any platform when using GNU C and not on any platform
when a different compiler is being used.

closes pandas-dev#14373

Author: aixtools

pandas/src/msgpack/unpack_template.h

@@ -17,7 +17,7 @@
  */
 
 #ifndef USE_CASE_RANGE
-#if !defined(_MSC_VER)
+#ifdef __GNUC__
 #define USE_CASE_RANGE
 #endif
 #endif