UTIL: Add workaround for libc++ for util::strings::convert<double>()

Behavior of parsing doubles from streams is different in LLVM's libc++
compared to GCC's libstdc++, see [1]. This causes
StringTest.StringConversionTest to fail when compiling against LLVM's
libc++:

  [ RUN      ] StringTest.StringConversionTest
  util/gtest_string.cc:48: Failure
  Expected equality of these values:
    10.1234
    util::string::convert<double>("10.1234asfd", false)
      Which is: 0

  [  FAILED  ] StringTest.StringConversionTest (0 ms)

This mainly affects macOS because it defaults to libc++.

The behavior of libstdc++ is what makes most sense here because
std::stringstream::operator>>() will call std::strtod() internally
which will extract 10.1234 from the string.

This is fixed by implementing a workaround for libc++.

[1] https://www.github.com/llvm/llvm-project/issues/18156

libs/util/string_utils.h

@@ -158,6 +158,26 @@ convert (std::string const& str, bool strict_conversion)
     return ret;
 }
 
+#if defined(_LIBCPP_VERSION)
+/* LLVM's libc++'s behavior when parsing a double from a stream is different
+ * than GCC's libstdc++, see the following issue:
+ *   https://www.github.com/llvm/llvm-project/issues/18156
+ *
+ * This is why the following workaround is needed when compiling against
+ * libc++.
+ */
+template <>
+inline double
+convert (std::string const& str, bool strict_conversion)
+{
+    std::size_t pos = 0;
+    double ret = std::stod(str, &pos);
+    if (strict_conversion && pos != str.length())
+        throw std::invalid_argument("Invalid string conversion: " + str);
+    return ret;
+}
+#endif
+
 template <typename T>
 inline char const*
 for_type (void)