build: Only allow ASCII identifiers

While emoji and other symbols in C++ identifers (as accepted by newer
compilers) are fun, they might create confusion during code review, for
example because some symbols look very similar. Forbid such extended
identifiers for now.

This is done by providing `-fno-extended-identifiers`. Thanks to sipa
for suggesting this compiler flag.

configure.ac

@@ -422,6 +422,9 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"],,[[$CXXFLAG_WERROR]])
 fi
 
+dnl Don't allow extended (non-ASCII) symbols in identifiers. This is easier for code review.
+AX_CHECK_COMPILE_FLAG([-fno-extended-identifiers],[[CXXFLAGS="$CXXFLAGS -fno-extended-identifiers"]],,[[$CXXFLAG_WERROR]])
+
 enable_sse42=no
 enable_sse41=no
 enable_avx2=no