X avoid fmt::vsprintf (now compils again with latest pygolang@master)

wcfs/client/wcfs_misc.cpp

@@ -224,6 +224,8 @@ tuple<uint64_t, error> parseUint(const string& s) {
 // xerr::
 namespace xerr {
 
+// XXX don't require fmt::vsprintf
+#if 0
 Contextf::Contextf(const char *format, ...) {
     Contextf& c = *this;
 
@@ -232,6 +234,7 @@ Contextf::Contextf(const char *format, ...) {
     c.errctx = fmt::vsprintf(format, argp);
     va_end(argp);
 }
+#endif
 
 error Contextf::operator() (error err) const {
     const Contextf& c = *this;

wcfs/client/wcfs_misc.h

@@ -146,6 +146,8 @@ typedef uint64_t Oid;
 }   // zodb::
 
 
+#include <golang/fmt.h>
+
 // xerr::
 namespace xerr {
 
@@ -164,11 +166,16 @@ class Contextf {
     string errctx;
 
 public:
-    Contextf(const char *format, ...);
+    template<typename ...Argv>
+    inline Contextf(const char *format, Argv... argv) {
+        // XXX string() to avoid "error: format not a string literal" given by -Werror=format-security
+        errctx = fmt::sprintf(string(format), argv...);
+    }
+
     error operator() (error) const;
 };
 
-}
+}   // xerr::
 
 
 // wcfs::