Fix over-writing library prologue

when linking the cprover library. The default mode for this constructor of
stringstream is to over-write the string used in construction, which presumably
was not the intention here.

Note: until now there wasn't anything too interesting in the prologue except for
the string-abstraction define, which was ignored. But we might want more defines
in the future.

Author: petr-bauch

src/ansi-c/cprover_library.cpp

@@ -45,7 +45,9 @@ std::string get_cprover_library_text(
   const struct cprover_library_entryt cprover_library[],
   const std::string &prologue)
 {
-  std::ostringstream library_text(prologue);
+  // the default mode is ios_base::out which means subsequent write to the
+  // stream will overwrite the original content
+  std::ostringstream library_text(prologue, std::ios_base::ate);
 
   std::size_t count=0;