Determine the compiler's default include and lib directories at qmake time

This fixes a long-standing issue for Qt packages, where the
paths detected at configure time do not necessarily match the
paths on the user's machine. Hence they have been stripped
manually from qconfig.pri so far, preventing moc from resolving
some includes.

The same logic in configure is left alone for the time being,
since the paths there are also used to filter paths returned
by pg_config and mysql_config. I expect that this will
eventually be removed too in a bigger refactoring going on
right now in dev.

Asking the compiler for implicit paths only works for non-msvc
builds - that is, gcc, clang and icc fortunately have a
compatible way to retrieve the paths. MSVC works
solely on environment variables, which will be taken into
account by a separate patch.

[ChangeLog][qmake] The implicit compiler directories that
moc needs for resolving include files are now determined
when qmake runs. So far QMAKE_DEFAULT_INCDIR was determin...