Process generator expressions for 'system' include directories.

Since commit 08cb4fa4 (Process generator expressions in the
INCLUDE_DIRECTORIES property., 2012-09-18), it is possible to use
generator expressions with the include_directories command.

As that command can also have a SYSTEM argument, ensure that the
result of using that argument with generator expressions gives a
sane result.
Deduplicate the isGeneratorExpression method.

This API seems like the most appropriate.
De-duplicate validation of genex target names.
Test printing origin of include dirs from tll().
The COMPATIBLE_INTERFACE does not affect the target it is set on.

Test and document this.
Ensure type specific compatible interface properties do not intersect.

Before, the boolean version would always win, and the string one would
be ignored.
Fix generation of COMPILE_DEFINITIONS in DependInfo.cmake.

As INTERFACE_COMPILE_DEFINITIONS are now possible, we can have
situations like this:

 add_library(foo ...)
 add_library(bar ...)
 target_link_libraries(foo bar)

 target_compile_definitions(bar INTERFACE SOME_DEF)

The INTERFACE_COMPILE_DEFINITIONS of bar determine how foo should be
compiled, and if they change, foo should be rebuilt.

Additionally, as of commit d1446ca7 (Append the COMPILE_DEFINITIONS
from the Makefile to all targets., 2012-09-17), we don't need to
read definitions from the makefile if we read them from the target,
so also de-duplicate the cached info.

The DependInfo for INTERFACE_INCLUDE_DIRECTORIES is already handled
correctly.
Merge topic 'minor-fixes' into next

b5f8a0c Fix generation of COMPILE_DEFINITIONS in DependInfo.cmake.
f333f14 Ensure type specific compatible interface properties do not intersect.
431c5a6 The COMPATIBLE_INTERFACE does not affect the target it is set on.
611c779 Test printing origin of include dirs from tll().
13965a7 De-duplicate validation of genex target names.
2e04459 Deduplicate the isGeneratorExpression method.
1714c27 Process generator expressions for 'system' include directories.