Remove extern declaration for the absorb_trace_exceptions flag

src/utils/error.hpp

@@ -1,7 +1,6 @@
 #ifndef ERROR_HPP
 #define ERROR_HPP
 
-#include <atomic>
 #include <exception>
 #include <string>
 #include <utility>
@@ -165,8 +164,6 @@ namespace detail {
      // Check condition in both debug. std::runtime_error on failure.
      #define ASSERT(...) PHONY_USE(__VA_ARGS__)
     #endif
-
-    extern std::atomic_bool absorb_trace_exceptions;
 }
 }