Generate build information from CHANGES.md

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).

Android.mk

@@ -33,6 +33,24 @@
 
 LOCAL_PATH := $(call my-dir)
 
+# Generate glslang/build_info.h
+GLSLANG_GENERATED_INCLUDEDIR:=$(TARGET_OUT)/include
+GLSLANG_BUILD_INFO_H:=$(GLSLANG_GENERATED_INCLUDEDIR)/glslang/build_info.h
+
+define gen_glslang_build_info_h
+$(call generate-file-dir,$(GLSLANG_GENERATED_INCLUDEDIR)/dummy_filename)
+$(GLSLANG_BUILD_INFO_H): \
+		$(LOCAL_PATH)/build_info.py \
+		$(LOCAL_PATH)/build_info.h.tmpl \
+		$(LOCAL_PATH)/CHANGES.md
+		@$(HOST_PYTHON) $(LOCAL_PATH)/build_info.py \
+						$(LOCAL_PATH) \
+						-i $(LOCAL_PATH)/build_info.h.tmpl \
+						-o $(GLSLANG_BUILD_INFO_H)
+		@echo "[$(TARGET_ARCH_ABI)] Generate       : $(GLSLANG_BUILD_INFO_H) <= CHANGES.md"
+endef
+$(eval $(call gen_glslang_build_info_h))
+
 GLSLANG_OS_FLAGS := -DGLSLANG_OSINCLUDE_UNIX
 # AMD and NV extensions are turned on by default in upstream Glslang.
 GLSLANG_DEFINES:= -DAMD_EXTENSIONS -DNV_EXTENSIONS -DENABLE_HLSL $(GLSLANG_OS_FLAGS)
@@ -55,18 +73,14 @@ LOCAL_C_INCLUDES:=$(LOCAL_PATH)/OGLCompiler
 LOCAL_STATIC_LIBRARIES:=OSDependent
 include $(BUILD_STATIC_LIBRARY)
 
-# Build Glslang's HLSL parser library.
+# Build the stubbed HLSL library.
+# The HLSL source is now directly referenced by the glslang static library
+# instead.
 include $(CLEAR_VARS)
 LOCAL_MODULE:=HLSL
 LOCAL_CXXFLAGS:=-std=c++11 -fno-exceptions -fno-rtti $(GLSLANG_DEFINES)
 LOCAL_SRC_FILES:= \
-		glslang/HLSL/hlslAttributes.cpp \
-		glslang/HLSL/hlslGrammar.cpp \
-		glslang/HLSL/hlslOpMap.cpp \
-		glslang/HLSL/hlslParseables.cpp \
-		glslang/HLSL/hlslParseHelper.cpp \
-		glslang/HLSL/hlslScanContext.cpp \
-		glslang/HLSL/hlslTokenStream.cpp
+	hlsl/stub.cpp
 LOCAL_C_INCLUDES:=$(LOCAL_PATH) \
 	$(LOCAL_PATH)/glslang/HLSL
 include $(BUILD_STATIC_LIBRARY)
@@ -74,12 +88,23 @@ include $(BUILD_STATIC_LIBRARY)
 include $(CLEAR_VARS)
 GLSLANG_OUT_PATH=$(if $(call host-path-is-absolute,$(TARGET_OUT)),$(TARGET_OUT),$(abspath $(TARGET_OUT)))
 
+# ShaderLang.cpp depends on the generated build_info.h
+$(LOCAL_PATH)/glslang/MachineIndependent/ShaderLang.cpp: \
+	$(GLSLANG_BUILD_INFO_H)
+
 LOCAL_MODULE:=glslang
 LOCAL_CXXFLAGS:=-std=c++11 -fno-exceptions -fno-rtti $(GLSLANG_DEFINES)
 LOCAL_EXPORT_C_INCLUDES:=$(LOCAL_PATH)
 LOCAL_SRC_FILES:= \
 		glslang/GenericCodeGen/CodeGen.cpp \
 		glslang/GenericCodeGen/Link.cpp \
+		glslang/HLSL/hlslAttributes.cpp \
+		glslang/HLSL/hlslGrammar.cpp \
+		glslang/HLSL/hlslOpMap.cpp \
+		glslang/HLSL/hlslParseables.cpp \
+		glslang/HLSL/hlslParseHelper.cpp \
+		glslang/HLSL/hlslScanContext.cpp \
+		glslang/HLSL/hlslTokenStream.cpp \
 		glslang/MachineIndependent/attribute.cpp \
 		glslang/MachineIndependent/Constant.cpp \
 		glslang/MachineIndependent/glslang_tab.cpp \
@@ -109,14 +134,19 @@ LOCAL_SRC_FILES:= \
 		glslang/MachineIndependent/preprocessor/PpTokens.cpp
 LOCAL_C_INCLUDES:=$(LOCAL_PATH) \
 	$(LOCAL_PATH)/glslang/MachineIndependent \
+	$(GLSLANG_GENERATED_INCLUDEDIR) \
 	$(GLSLANG_OUT_PATH)
 LOCAL_STATIC_LIBRARIES:=OSDependent OGLCompiler HLSL
 include $(BUILD_STATIC_LIBRARY)
 
 include $(CLEAR_VARS)
+
+# GlslangToSpv.cpp depends on the generated build_info.h
+$(LOCAL_PATH)/SPIRV/GlslangToSpv.cpp: \
+	$(GLSLANG_BUILD_INFO_H)
+
 LOCAL_MODULE:=SPIRV
 LOCAL_CXXFLAGS:=-std=c++11 -fno-exceptions -fno-rtti -Werror $(GLSLANG_DEFINES)
-LOCAL_EXPORT_C_INCLUDES:=$(LOCAL_PATH)
 LOCAL_SRC_FILES:= \
 	SPIRV/GlslangToSpv.cpp \
 	SPIRV/InReadableOrder.cpp \
@@ -127,7 +157,9 @@ LOCAL_SRC_FILES:= \
 	SPIRV/SpvTools.cpp \
 	SPIRV/disassemble.cpp \
 	SPIRV/doc.cpp
-LOCAL_C_INCLUDES:=$(LOCAL_PATH) $(LOCAL_PATH)/glslang/SPIRV
+LOCAL_C_INCLUDES:=$(LOCAL_PATH) \
+	$(LOCAL_PATH)/glslang/SPIRV \
+	$(GLSLANG_GENERATED_INCLUDEDIR)
 LOCAL_EXPORT_C_INCLUDES:=$(LOCAL_PATH)/glslang/SPIRV
 LOCAL_STATIC_LIBRARIES:=glslang
 include $(BUILD_STATIC_LIBRARY)