configure: workaround "maybe uninitialized" warning

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At least g++ 15.2.0 and 15.2.1 generate warnings about "maybe uninitialized" in std::string/std::basic_string class as described in #3709. It is most probably a false positive as this warning has been seen before.

This PR "fixes" #3709 by detecting the warning in a configure test and adds "-Wno-maybe-uninitialized" to the compiler invocation if a warning was present. Older g++, like from debian 11, 12 and 13 as well as clang++, will not have this warning and no additional compiler flags are added.
The configure test will no longer flag a g++ when the distribution's compiler is updated with the issue fixed. That makes the configure test the better choice than to use complex #if constructs in the sources.

If it was only g++, then the test could have been slightly more tight by using "-Werror=maybe-uninitialized", but that does not work with clang++. Therefore, it is assumed that no other warnings will occur in the test and that should not be the case in the test program:

#include <boost/python/extract.hpp>
std::string func(boost::python::object& o) {
	return boost::python::extract<std::string>(o);
}
int main() { return 0; }