use fmt::format while clang doesn't support std::format yet

Index: rts/System/Matrix44f.h
--- rts/System/Matrix44f.h.orig
+++ rts/System/Matrix44f.h
@@ -173,7 +173,7 @@ class CMatrix44f (public)
 	};
 
 	std::string str() const {
-		return std::format(
+		return fmt::format(
 			"m44(\n{:.3f} {:.3f} {:.3f} {:.3f}\n{:.3f} {:.3f} {:.3f} {:.3f}\n{:.3f} {:.3f} {:.3f} {:.3f}\n{:.3f} {:.3f} {:.3f} {:.3f})",
 			m[0], m[4], m[8], m[12], m[1], m[5], m[9], m[13], m[2], m[6], m[10], m[14], m[3], m[7], m[11], m[15]);
 	}
