Create snapshots for buildsystem directories during configure time.
This class will be extended in follow up commits to snapshot
all values in the cmState.
At this point, it is an interface to the cache. It will be extended
to be a universal interface for access to and manipulation of
configuration-time data (defintions, properties on targets,
directories, source files etc).
This will allow porting all command implementations away
from the cmMakefile and cmTarget classes, and result in something
more-purely related to configuration-time processing of cmake
commands. That should serve at least the following goals:
* Split the CMake implementation more definitively into three
stages: Configuration, computation and generation, and be able to
implement each optimally for memory access patterns etc.
* Make better IDE integration possible by making more configuration
data available.
* Make it possiblte to use a smaller library than CMakeLib.a in
cpack and ctest, resulting in smaller executables.
* Make it possible to run the configure step multiple times in
the same CMake run (#14539).
Manage its lifetime in the cmake class, and add a convenience accessor
to cmMakefile.
Stephen Kelly