[build] compiles aegisub in C++17 whitout any issue

configure.ac

@@ -194,8 +194,8 @@ AC_ARG_ENABLE(compiler-flags, AS_HELP_STRING([--disable-compiler-flags],[Disable
 AS_IF([test x$enable_compiler_flags != xno], [
   CFLAGS="$CFLAGS -Wall -Wextra -Wno-unused-parameter -std=gnu99 -pipe -g"
   CXXFLAGS="$CXXFLAGS -Wall -Wextra -Wno-unused-parameter -fno-strict-aliasing -pipe -g"
-  AC_CXX_FLAG([-std=c++14])
-  AC_CXX_FLAG([-Wno-c++14-narrowing])
+  AC_CXX_FLAG([-std=c++17])
+  AC_CXX_FLAG([-Wno-c++17-narrowing])
   AC_C_FLAG([-Wno-unused-local-typedefs])
   AC_CXX_FLAG([-Wno-unused-local-typedefs])