eliminate warnings in linux mingw build

win32/pal_config.h

@@ -57,4 +57,6 @@
 
 #define PAL_FILESYSTEM_IGNORE_CASE 1
 
+#ifndef PATH_MAX
 #define PATH_MAX MAX_PATH
+#endif

winrt/pal_config.h

@@ -54,7 +54,9 @@
 
 #define PAL_FILESYSTEM_IGNORE_CASE 1
 
+#ifndef PATH_MAX
 #define PATH_MAX MAX_PATH
+#endif
 
 PAL_C_LINKAGE_BEGIN