define HOST_NAME_MAX for Windows and Mac

Summary:
This page says it can't be greater than 256:
https://docs.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-gethostname

POSIX one is equal to 255, so set it to 255.

Reviewed By: yfeldblum

Differential Revision: D27370716

fbshipit-source-id: 458ebc02d2914a0422862fafedd089e0c13c5e6d

folly/portability/Stdlib.h

@@ -24,6 +24,7 @@
 #if __has_include(<crt_externs.h>)
 #include <crt_externs.h> // @manual
 #endif
+#define HOST_NAME_MAX _POSIX_HOST_NAME_MAX
 #endif
 
 extern "C" {
@@ -36,6 +37,7 @@ extern "C" {
 #define PATH_MAX _MAX_PATH
 #define MAXPATHLEN _MAX_PATH
 #define NAME_MAX _MAX_FNAME
+#define HOST_NAME_MAX 255
 
 char* mktemp(char* tn);
 char* mkdtemp(char* tn);