contrib: configure-for-system: use meson by default and allow specifying the build directory

With the deprecation of autotools, use meson by default. For the moment, it's still possible to build with autotools passing -a/--autotools.

Instead of forcing to use ./build directory, let's make the the user specify what directory he wants to use. This will allow to have multiple build directories with different configurations as meson is designed to allow, without having to overwrite the existing build one.

Additionally, as we allow to specify different build directories other than './build', let's not asume that the user wants to overwrite it by default. Instead, the script will asume ./build if the user doesn't specify the build directory, but only if it doesn't exist. If it does, the user will have to force overwritting it with --meson ./build.