This PR significantly reworks the way glslang is versioned.
Instead of committing changes to the `GLSLANG_MINOR_VERSION` define in
`glslang/Public/ShaderLang.h`, and using `make-revision` to generate
`GLSLANG_PATCH_LEVEL` in `glslang/Include/revision.h`, all version
information is now derived from the new `CHANGES.md` file.
`CHANGES.md` acts as the single source of truth for glslang version
information, along with a convenient place to put all release notes for
each notable change made.
`CHANGES.md` is parsed using the new `build_info.py` python script.
This script can read basic template files to produce new source files,
which it does to read the new `build_info.h.tmpl` to generate (at build
time) a glslang private header at
`<build-dir>/include/glslang/build_info.h`.
I've written generators for each of the CMake, Bazel, gn, and
`Android.mk` build scripts.
The new version code conforms to the Semantic Versioning 2.0 spec.
This new version is also used by the CMake rules to produce versioned
shared objects, including a major-versioned SONAME.
New APIs:
---------
* `glslang::GetVersion()` returns a `Version` struct with the version
major, minor, patch and flavor.
Breaking API changes:
---------------------
* The public defines `GLSLANG_MINOR_VERSION` and `GLSLANG_PATCH_LEVEL`
have been entirely removed.
* `glslang/Public/ShaderLang.h` and `glslang/Include/revision.h` have
been deleted.
* Instead, `<build-dir>/include/glslang/build_info.h` is created in
the build directory, and `<build-dir>/include` is a CMake `PUBLIC`
(dependee-inherited) include directory for the glslang targets.
* `<build-dir>/include/glslang/build_info.h` contains the following
new #defines:
`GLSLANG_VERSION_MAJOR`, `GLSLANG_VERSION_MINOR`,
`GLSLANG_VERSION_PATCH`, `GLSLANG_VERSION_FLAVOR`,
`GLSLANG_VERSION_GREATER_THAN(major, minor, patch)`,
`GLSLANG_VERSION_GREATER_OR_EQUAL_TO(major, minor, patch)`,
`GLSLANG_VERSION_LESS_THAN(major, minor, patch)`,
`GLSLANG_VERSION_LESS_OR_EQUAL_TO(major, minor, patch)`
* The CMake install output directory contains a copy of
`build_info.h` at: `include/glslang/build_info.h`
* Python3 is now always required to build glslang (likely always
required for transitive dependency builds).
