add helper function to set 'interruptThrown'

this fixes a linker failure on cygwin 64 due to some bad
interaction between tls and shared libraries.

see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64697

src/libutil/util.hh

@@ -267,7 +267,7 @@ void closeOnExec(int fd);
 
 extern bool _isInterrupted;
 
-extern thread_local bool interruptThrown;
+void setInterruptThrown();
 
 void _interrupted();