Try to autodetect and auto-enable compiler compatibility mode.

I'm sick of the neverending issue reports. Now the user doesn't need to
enable it in most cases (except when e.g. using Clang with
libstdc++-4.7, which can't be detected at all).
Don't display irrelevant compatibility messages.

When GCC46_COMPATIBILITY was passed explicitly, it printed out message
about autodetecting GCC 4.7, which is wrong.
package: remove explicit compatibility flags from Jenkins CI configs.