build: improve detection and reporting of -std= flag detection

First, reintroduce testing $CXX without any -std flag.
Second, abort configure if no usable combination was found.

References: GXH-106

configure.ac

@@ -24,29 +24,34 @@ my_CPPFLAGS="-D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64"
 my_CFLAGS="-Wall -Wmissing-declarations -Wwrite-strings"
 my_CXXFLAGS="-Wall -Wmissing-declarations"
 my_LDFLAGS=""
-AC_MSG_CHECKING([available C++ standard])
-cxxmode=""
-for i in "c++26" "c++2c" "c++23" "c++2b" "c++20"; do
+cxxmode="error"
+AC_MSG_CHECKING([for C++ standard availability])
+echo ""
+for i in "" "c++26" "c++2c" "c++23" "c++2b" "c++20"; do
 	dnl cov-scan cannot take too high a standard
 	AS_IF([test -n "$COVERITY" && test "$i" != "c++20"], [continue])
-	CXXFLAGS="$saved_CXXFLAGS -std=$i"
+	AC_MSG_CHECKING([if $CXX${i:+ -std=$i} works for our code])
+	CXXFLAGS="$saved_CXXFLAGS${i:+ -std=$i}"
 	AC_COMPILE_IFELSE([AC_LANG_SOURCE([
 		#include <span>
 		#include <vector>
 		int main() {
 			std::vector<int> v{1,9};
 			std::span s = v;
 		}
-	])], [cxxmode="$i"])
-	AS_IF([test -n "$cxxmode"], [break])
+	])], [
+		AC_MSG_RESULT([yes])
+		cxxmode="${i:+ -std=$i}"
+		break
+	], [
+		AC_MSG_RESULT([no])
+	])
 done
 CXXFLAGS="$saved_CXXFLAGS"
-AS_IF([test -n "$cxxmode"], [
-	my_CXXFLAGS="$my_CXXFLAGS -std=$cxxmode"
-	AC_MSG_RESULT([$cxxmode])
-], [
-	AC_MSG_RESULT([none])
+AS_IF([test "$cxxmode" = error], [
+	AC_MSG_ERROR([None of the -std= flags we tried led to successful compilation, but we need C++20 support.])
 ])
+my_CXXFLAGS="$my_CXXFLAGS${cxxmode}"
 
 AC_ARG_WITH([asan], AS_HELP_STRING([--with-asan], [Activate Address Sanitizer]),
 	[