Don't load Clang-CXX from AppleClang-CXX.

The Clang-CXX module is going to get version-specific checks, and
the version system for AppleClang is not the same as Clang.
Add some COMPILE_OPTIONS for specifying C++ dialect.

These are compiler-specific, compiler version specific, extension specific
and standard version specific.
cmTarget: Add CXX_STANDARD and CXX_EXTENSION target properties.

These are used to determine whether to add -std=c++11, -std=gnu++11
etc flags on the compile line.
Merge branch 'fix-Qt5-non-Windows' into release
Merge branch 'add_custom_command-no-INTERFACE-lib' into release
cmGeneratorTarget: Compute the object directory early.

Ensure it is populated before tracing dependencies.
cmTarget: Use GetSourceFiles for languages.
cmTarget: Rename AddSource method for backward compatibility.

Add a new AddSource method for future use.
cmSourceFileLocation: Make copyable and assignable.

This allows using it in containers and algorithms.
cmTarget: Add a method to obtain list of filenames for sources.
cmTarget: Use string API to add sources to cmTarget objects.

Continue to call GetOrCreateSource where necessary to create
cmSourceFile objects which have the GENERATED attribute set.
cmTarget: Store strings instead of cmSourceFile* to represent SOURCES.

This will allow the strings to contain generator expressions.

At this point, generator expressions are still not part of the
SOURCES property when it is read.
cmTarget: Remove AddSourceFile method

It is no longer used.
Genex: Evaluate TARGET_OBJECTS as a normal expression.
cmStringCommand: Add GENEX_STRIP subcommand.

Strip out any generator expressions in the input string.
cmGlobalGenerator: Add interface to call ForceLinkerLanguages

Avoid calling it too early when cmGeneratorTarget instances don't
yet exist.
cmTarget: Include TARGET_OBJECTS genex in target SOURCES property.

Add policy CMP0051 to control this behavior.
cmTarget: Allow any generator expression in SOURCES property.

Remove use of UseObjectLibraries from Makefile and Ninja generators. It
is not needed now because those generators use GetExternalObjects
which already contains the objects from object libraries.

The VS10 generator calls both the UseObjectLibraries and the GetExternalObjects
methods. Ensure that duplicates are not created by skipping objects
from object libraries in handling of GetExternalObjects.

Similarly, fix Xcode object handling by skipping external objects from
OBJECT_LIBRARY usage as appropriate.

The error message in the BadSourceExpression1 test is now reported
by the generator expression evaluator, so it has different text.
cmComputeTargetDepends: Track object library depends.

Relieve cmGeneratorTarget of that responsibility.
cmGeneratorTarget: Compute consumed object libraries on demand.

Remove up-front object library computation from cmGlobalGenerator.

Adjust tests for message coming from the generator expression
evaluation.