Project: Don't require computed default dialect if compiler was forced.

Commit 7235334a (Project: Determine default language dialect for the
compiler., 2015-09-15) introduced a mechanism to determine the default
dialect used for the running compiler.  If conditions in
the <CompilerId>-<Lang>.cmake file are such that compile features for
that version of the compiler should be supported, the _DEFAULT_STANDARD
is set to the computed value.

However, the CMakeForceCompiler module allows users to bypass execution of the
compiler by CMake.  In that case, do not set the _DEFAULT_STANDARD variable at
all, which effectively disables the compile-features where the module is used.

No compile features have ever been recorded where the module is used so no
functionality is lost.