Fix name manling warning on Ubuntu 18

Caused by `std::toupper` being declared with the deprecated `throw ()`
in the standard library and the meaning of this changing between C++
versions.

Author: thomasspriggs

src/solvers/smt2_incremental/smt_response_validation.cpp

@@ -230,7 +230,8 @@ static optionalt<smt_termt> valid_smt_hex(const std::string &text)
   const std::string hex{text.begin() + 2, text.end()};
   // SMT-LIB 2 allows hex characters to be upper of lower case, but they should
   // be upper case for mp_integer.
-  const mp_integer value = string2integer(make_range(hex).map(::toupper), 16);
+  const mp_integer value =
+    string2integer(make_range(hex).map<std::function<int(int)>>(toupper), 16);
   const std::size_t width = (text.size() - 2) * 4;
   return {smt_bit_vector_constant_termt{value, width}};
 }