Automatically define MONO_DISABLE_GSHAREDVT variable when gsharedvt isn't enabled

Automatically define MONO_DISABLE_GSHAREDVT variable when gsharedvt isn't enabled

Before when the variable wasn't set (= the default), the mobile_static profile would try to pass
the -O=gsharedvt flag even when gsharedvt is not enabled (e.g. when building without mono-extensions)
which causes AOTing to fail.

A better solution is to export the variable from configure.ac into the mcs build system depending
on whether gsharedvt is enabled at configure time.