Always consider includes from IMPORTED targets to be SYSTEM.

Introduce a target property to control this behavior variable
to set the default value for the target property.

This does not affect try_compile runs.
Tests: Make find_package(Qt4) QUIET

The lack of Qt4 on a system should silently skip the corresponding tests
with no other messages.  This is already the case for other find_package
calls in Tests/CMakeLists.txt.
cmake: Split -E command implementation into separate source file

Move the cmake::ExecuteCMakeCommand static method and all the static
methods it calls out of the 'cmake' class to a separate 'cmcmd' class.
Build the latter as part of the main cmake executable with cmakemain.cxx
and not in CMakeLib.  Drop unused header includes from "cmake.cxx".

By moving this implementation out of cmake.cxx we avoid carrying it
around in all the executables that use class 'cmake'.  It is needed only
for the main "cmake -E" functionality.
Merge topic 'IMPORTED-target-SYSTEM-includes'

a63fcbc Always consider includes from IMPORTED targets to be SYSTEM.
Merge topic 'test-Qt4-quiet'

a3170c8 Tests: Make find_package(Qt4) QUIET
Merge topic 'split-cmake-E'

c04995b cmake: Split -E command implementation into separate source file