Add a -libexecdir option to the configures

Add a -libexecdir option to the configures

User applications are those that users run directly, whether it be for
development or not. The executable binaries that the user does not
usually run but is still required for proper functioning are called
"program executables" in Autoconf and they are placed in libexec.

This commit adds support for "program executables" in Qt by adding the
-libexecdir option to the configures, the qmake variable
QT_INSTALL_LIBEXECS (note the plural, to match all other properties),
and QLibraryInfo::LibraryExecutables.

At the time of this commit, the only expected "program executable" is
the QtWebProcess, the WebKit2 helper process from QtWebKit.

Change-Id: I66c3a3e0cf7f9d93b5f88f55f18e957faff608fc
Reviewed-by: Lars Knoll <lars.knoll@digia.com>