Ensure HAS_STRPTIME is defined by default

Builds now always have a default value of 0 or 1 for HAS_STRPTIME,
but it can be overridden on the commandline.

This is to address -Wundef warnings, and the feedback from google#330.

Author: derekmauro

src/time_zone_format.cc

@@ -15,10 +15,12 @@
 #if !defined(HAS_STRPTIME)
 # if !defined(_MSC_VER) && !defined(__MINGW32__) && !defined(__VXWORKS__)
 #  define HAS_STRPTIME 1  // Assume everyone else has strptime().
+# else
+#  define HAS_STRPTIME 0
 # endif
 #endif
 
-#if defined(HAS_STRPTIME) && HAS_STRPTIME
+#if HAS_STRPTIME
 # if !defined(_XOPEN_SOURCE) && !defined(__FreeBSD__) && !defined(__OpenBSD__)
 #  define _XOPEN_SOURCE 500  // Exposes definitions for SUSv2 (UNIX 98).
 # endif
@@ -39,7 +41,7 @@
 #include <limits>
 #include <string>
 #include <vector>
-#if !defined(HAS_STRPTIME)
+#if !HAS_STRPTIME
 #include <iomanip>
 #include <sstream>
 #endif
@@ -52,7 +54,7 @@ namespace detail {
 
 namespace {
 
-#if !defined(HAS_STRPTIME)
+#if !HAS_STRPTIME
 // Build a strptime() using C++11's std::get_time().
 char* strptime(const char* s, const char* fmt, std::tm* tm) {
   std::istringstream input(s);