Always enable exceptions when building with clang.

Passing -fdelayed-template-parsing to clang to work around the issue of code
that includes non-instantiated templates that throw exceptions causes a lot
of problems, as described in
http://lists.kde.org/?l=kde-core-devel&m=138157459706783&w=2.

Instead of adding workarounds, just enable exceptions by default and live
with the fact that the generated code will be slightly larger.

CCMAIL: kde-buildsystem@kde.org