Fixes warning when redefining PATH_MAX when building with MINGW.

Fixes warning thrown when compiling digest.cc with MinGW64, by only defining PATH_MAX, if it has not yet been defined.
Else building with MinGW64, throws the following warning:

<PATH_TO_SOURCE_FOLDER>\boringssl\src\tool\digest.cc:39: warning: "PATH_MAX" redefined
   39 | #define PATH_MAX MAX_PATH
      |
In file included from C:/msys64/mingw64/lib/gcc/x86_64-w64-mingw32/9.3.0/include-fixed/limits.h:194,
                 from C:/msys64/mingw64/lib/gcc/x86_64-w64-mingw32/9.3.0/include-fixed/syslimits.h:7,
                 from C:/msys64/mingw64/lib/gcc/x86_64-w64-mingw32/9.3.0/include-fixed/limits.h:34,
                 from C:/msys64/mingw64/x86_64-w64-mingw32/include/pthread.h:67,
                 from C:/msys64/mingw64/include/c++/9.3.0/x86_64-w64-mingw32/bits/gthr-default.h:35,
                 from C:/msys64/mingw64/include/c++/9.3.0/x86_64-w64-mingw32/bits/gthr.h:148,
                 from C:/msys64/mingw64/include/c++/9.3.0/ext/atomicity.h:35,
                 from C:/msys64/mingw64/include/c++/9.3.0/memory:73,
                 from <PATH_TO_SOURCE_FOLDER>/boringssl/src/include/openssl/base.h:473,
                 from <PATH_TO_SOURCE_FOLDER>\boringssl\src\tool\digest.cc:15:
C:/msys64/mingw64/x86_64-w64-mingw32/include/limits.h:20: note: this is the location of the previous definition
   20 | #define PATH_MAX 260
      |

Change-Id: I29eb33ee8fad9e4e80d9348a0d5e4057dfac620c
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/41705
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
diff --git a/tool/digest.cc b/tool/digest.cc
index 4adaf8d..3c8fd5a 100644
--- a/tool/digest.cc
+++ b/tool/digest.cc
@@ -36,8 +36,10 @@
 #include <windows.h>
 OPENSSL_MSVC_PRAGMA(warning(pop))
 #include <io.h>
+#if !defined(PATH_MAX)
 #define PATH_MAX MAX_PATH
 #endif
+#endif
 
 #include <openssl/digest.h>