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

**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:

```cmake
target_include_directories(SPIRV PUBLIC
$
$)
```
`CMAKE_CURRENT_SOURCE_DIR` in `SPIRV/CMakeLists.txt` is `/SPIRV/`. This means the BUILD interface adds `/` to include paths.
With that, `glslang/SPIRV/GlslangToSpv.h` resolves to `/glslang/SPIRV/GlslangToSpv.h`, which does not exist in the source tree. The actual file is at `/SPIRV/GlslangToSpv.h`.

The INSTALL interface correctly places headers at `/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:

```cmake
...
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()
...
```

Start with SPIRV/CMakeLists.txt and the SPIRV target's BUILD and INSTALL include directories, then configure a minimal CMake subproject that includes glslang/SPIRV/GlslangToSpv.h. Done means the include compiles from the build tree and remains consistent with the installed header layout.

Written by the indexing model from the issue text.