Libpqxx comes with support for two different build systems: the GNU autotools, and CMake.
We need to teach both of these to test things like "does this compiler
environment support std::to_chars for floating-point types?"
In both build systems we test these things by trying to compile a particular snippet of code, found in this directry, and seeing whether that succeeds.
To avoid duplicating those snippets for multiple build systems, we put them here. Both the autotools configuration and the CMake configuration can refer to them that way. We generate autoconf and cmake configuration automatically to inject those checks, avoiding tedious repetition.
Some of the checks are based on C++20 feature test macros. We generate those
automatically using tools/generate_cxx_checks.py.
It took a bit of nasty magic to read a C++ source file into m4 for the autoconf side and treat it as a string literal, without macro expansion. There is every chance that I missed something, so be prepared for tests failing for unexpected reasons! Some C++ syntax may end up having an unforeseen meaning in m4, and screw up the handling of the code snippet. Re-configure, and read your logs carefully after editing these snippets.