CoinTime: replace C++17 inline variable with C++11 static local

coinWallclockStart was declared as an inline variable (C++17), causing
build failures on MSVC when compiled without /std:c++17:

  CoinTime.hpp(94): error C7525: inline variables require at least '/std:c++17'

Replace with a static local variable inside CoinWallclockTime(), which
is guaranteed to be initialised exactly once and thread-safely under
C++11, without requiring any build-system changes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Haroldo

src/CoinTime.hpp

@@ -88,15 +88,13 @@ inline double CoinGetTimeOfDay()
 
 #endif // _MSC_VER
 
-/// Wall-clock epoch seconds at static-initialisation time (before main()).
-/// Defined as an inline variable (C++17) so every translation unit shares
-/// the same object and the value is fixed exactly once.
-inline const double coinWallclockStart = CoinGetTimeOfDay();
-
-/// Returns elapsed wall-clock time in seconds since program start.
+/// Returns elapsed wall-clock time in seconds since the first call.
+/// Uses a static local variable (C++11) so the start time is captured
+/// exactly once, thread-safely, without requiring C++17 inline variables.
 inline double CoinWallclockTime()
 {
-  return CoinGetTimeOfDay() - coinWallclockStart;
+  static const double wallclockStart = CoinGetTimeOfDay();
+  return CoinGetTimeOfDay() - wallclockStart;
 }
 
 //#############################################################################