Based on excellent comment in #1489, the logic
to set cmake policies was simplified. This will
provide an easier mechanism for maintaining the
conditional logic across many versions of cmake.
Automatically extract version information
from the zstd.h file. Use naming of variables
consisent with modern cmake and https://semver.org/
(Semantic Versioning 2.0.0, MAJOR, MINOR, PATCH)
Modern versions of cmake provide consistent
paradigms for configuring project external
interface values.
This set of changes provide a back port of
some of cmake 3+ paradigms back to cmake 2.8.9.
Set and allow use of the current cmake policies
for newer versions of cmake when available to
allow for modern compiler features to be
utilized when available.
NOTE: The intent is that future modifications to
cmake will enable (conditional on cmake version support)
the ability to support modern linkage, and target
export mechanisms. Those future changes will
make incorporating zstd into other packages
much easier.
This patch also allows the more rigourous error
checking of commmon cmake errors to be preformed
by cmake (i.e. more stringent syntax checking and
create errors for common hard to find misuses of
cmake variables).
This patch also provides support for modern
compiler support options by cmake (like
enabling interprocedural optimization
if link time optimizations are known to be supported
by the compiler envirionment. IPO can be supported
by setting the CMAKE_INTERPROCEDURAL_OPTIMIZATION variable
for newer versions of cmake.
The CMAKE_BUILD_TYPE variable is a CACHE variable
and should be set in a way that persists and is
documented in the CACHE. Also set the default
values for the gui to ease selection of types.
These changes provide better support for GUI
configurators that support cmake.
Ancient versions of CMake required else(), endif(), and similar block
termination commands to have arguments matching the command starting the block.
This is no longer the preferred style.
