Added auxv-based VSX detection method for PowerPC and enabled it by default

contrib/powerpc-vsx/linux_aux.c

@@ -0,0 +1,34 @@
+/* contrib/powerpc-vsx/linux.c
+ *
+ * Copyright (c) 2016 Glenn Randers-Pehrson
+ * Written by Vadim Barkov, 2017.
+ *
+ * This code is released under the libpng license.
+ * For conditions of distribution and use, see the disclaimer
+ * and license in png.h
+ *
+ * SEE contrib/powerpc-vsx/README before reporting bugs
+ *
+ * STATUS: COMPILED
+ * BUG REPORTS: png-mng-implement@sourceforge.net
+ *
+ * png_have_vsx implemented for Linux by reading the widely available
+ * pseudo-file /proc/cpuinfo. Result is cached so if function will be called
+ * multiple times only one reading is perfomed.
+ *
+ * This code is strict ANSI-C and is probably moderately portable; it does
+ * however use <stdio.h> and it assumes that /proc/cpuinfo is never localized.
+ */
+
+#include "sys/auxv.h"
+#include "png.h"
+
+static int
+png_have_vsx(png_structp png_ptr)
+{
+   const unsigned long auxv = getauxval( AT_HWCAP );
+   if(auxv & (PPC_FEATURE_HAS_ALTIVEC|PPC_FEATURE_HAS_VSX ))
+	return 1;
+   else
+	return 0;
+}

powerpc/powerpc_init.c

@@ -32,7 +32,7 @@
  */
 #ifndef PNG_POWERPC_VSX_FILE
 #  ifdef __linux__
-#     define  PNG_POWERPC_VSX_FILE "contrib/powerpc-vsx/linux.c"
+#     define  PNG_POWERPC_VSX_FILE "contrib/powerpc-vsx/linux_aux.c"
 #  endif
 #endif