Describe the bug

Starting with 30170d6, the library uses memset_s for the secure string allocator.

This is problematic, because:

• not all compilers implement memset_s
• the commit suggests this fixes a c++20 issue, but the meson (and cmake) file's c++ standard selection is not incremented from c++11

Apart from that, the unit tests won't build because many spots the test code tries to convert between std::string and the custom allocator string.

To Reproduce
Steps to reproduce the behavior:

1. activate JSONCPP_USING_SECURE_MEMORY
2. build the code

Expected behavior

The code should compile with any C++ compiler.
The tests should compile regardless of the string allocator.

Desktop (please complete the following information):

• OS: NixOS
• Meson version 0.57.1
• Ninja version 1.10.2

Additional context
Add any other context about the problem here.