Replace non-threadsafe gmtime and setlocale

Make DateTimeStrFormat use boost::posix_time.

Also re-enable the util_DateTimeStrFormat tests, as they are no
longer platform specific.

src/util.cpp

@@ -14,6 +14,8 @@
 
 #include <stdarg.h>
 
+#include <boost/date_time/posix_time/posix_time.hpp>
+
 #ifndef WIN32
 // for posix_fallocate
 #ifdef __linux_
@@ -1400,3 +1402,13 @@ void SetupEnvironment()
     }
     #endif
 }
+
+std::string DateTimeStrFormat(const char* pszFormat, int64_t nTime)
+{
+    // std::locale takes ownership of the pointer
+    std::locale loc(std::locale::classic(), new boost::posix_time::time_facet(pszFormat));
+    std::stringstream ss;
+    ss.imbue(loc);
+    ss << boost::posix_time::from_time_t(nTime);
+    return ss.str();
+}