Fix build failure on glibc-2.27 caused by pow64 symbol collision.

https://github.com/LibRaw/LibRaw/pull/115
https://bugs.gentoo.org/647556
diff --git a/internal/dcraw_common.cpp b/internal/dcraw_common.cpp
index 18bcdbc..8cec744 100644
--- a/internal/dcraw_common.cpp
+++ b/internal/dcraw_common.cpp
@@ -5770,4 +5770,5 @@ static float powf_lim(float a, float b, float limup)
   return (b>limup || b < -limup)?0.f:powf(a,b);
 }
+#define powf64 powf_lim64 /* workaround symbol collision */
 static float powf64(float a, float b)
 {