Libpqxx comes with support for different build systems: the GNU autotools, CMake, Visual Studio's "nmake", and raw GNU "make" on Windows.
For several of these build systems, we need to test things like "does this
compiler environment support std::to_chars for floating-point types?"
We test these things by trying to compile a particular snippet of code, 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.
It took a bit of nasty magic to read a C++ source file into m4 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.