Build interface include path for `glslang/SPIRV/*` inconsistent with install layout

[LDAP]

glslang version: 16.2.0

Description

When glslang is used as a cmake subproject (e.g. via add_subdirectory or meson's cmake subproject integration), #include "glslang/SPIRV/GlslangToSpv.h" fails to compile because the header cannot be found. The SPIRV cmake target declares:

  target_include_directories(SPIRV PUBLIC
      $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
      $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)

CMAKE_CURRENT_SOURCE_DIR in SPIRV/CMakeLists.txt is <root>/SPIRV/. This means the BUILD interface adds <root>/ to include paths.
With that, glslang/SPIRV/GlslangToSpv.h resolves to <root>/glslang/SPIRV/GlslangToSpv.h, which does not exist in the source tree. The actual file is at <root>/SPIRV/GlslangToSpv.h.

The INSTALL interface correctly places headers at <prefix>/include/glslang/SPIRV/GlslangToSpv.h, so glslang/SPIRV/GlslangToSpv.h works for installed glslang.

The two interfaces are inconsistent.

Expected behaviour

#include "glslang/SPIRV/GlslangToSpv.h" should compile when glslang is used as a cmake subproject, consistent with the installed layout.

Suggested fix

Either add a glslang/SPIRV/ directory (or symlink) to the source tree mirroring the install layout, or adjust the BUILD interface include path so that glslang/SPIRV/GlslangToSpv.h resolves correctly from the source tree.

I patched the CMakeLists.txt like to fix the issue on by Ubuntu and Windows CIs:

...
if(ENABLE_SPIRV)
    add_subdirectory(SPIRV)
    # @LDAP: create glslang/SPIRV/ symlink in the generated include dir so that
    # #include "glslang/SPIRV/GlslangToSpv.h" works consistently with the install layout.
    file(MAKE_DIRECTORY "${GLSLANG_GENERATED_INCLUDEDIR}/glslang")
    file(CREATE_LINK "${CMAKE_CURRENT_SOURCE_DIR}/SPIRV" "${GLSLANG_GENERATED_INCLUDEDIR}/glslang/SPIRV" SYMBOLIC COPY_ON_ERROR)
endif()
...