fix: build failures on CMake 4.x and OpenCL headers >= 3.0.16

[rginon314]

Build Failures on Modern Toolchains (CMake ≥ 4.0 + OpenCL ≥ 3.0.16)

Summary

Building libfreenect2 from source fails with two independent errors when using
CMake 4.x and a recent OpenCL ICD loader (opencl-icd-loader ≥ 3.0.16 / Khronos
vendor headers that include cl_ext.h ≥ 3.0.16).

---

Environment

Component	Version
OS	Manjaro Linux (rolling, Arch-based)
CMake	4.2.3
GCC	15.2.1
OpenCL headers	via opencl-headers (cl_ext.h defining CL_ICDL_VERSION)
libusb	1.0.x
libfreenect2	commit fd64c5d (current HEAD of master)

---

Error 1 — cmake_minimum_required version too old (CMake ≥ 4.0)

Symptom

CMake Error at CMakeLists.txt:1 (CMAKE_MINIMUM_REQUIRED):
  Compatibility with CMake < 3.5 has been removed from CMake.
  Update the VERSION argument <min> value.

Root cause

Three CMakeLists.txt files declare cmake_minimum_required(VERSION 2.8.12.1),
which CMake 4.0+ explicitly rejects.

Affected files:

• CMakeLists.txt
• examples/CMakeLists.txt
• tools/streamer_recorder/CMakeLists.txt

Fix

# Before
CMAKE_MINIMUM_REQUIRED(VERSION 2.8.12.1)

# After
cmake_minimum_required(VERSION 3.5)

---

Error 2 — CL_ICDL_VERSION macro collision (OpenCL headers ≥ 3.0.16)

Symptom

src/opencl_depth_packet_processor.cpp:254:15:
  error: expected unqualified-id before numeric constant
    const int CL_ICDL_VERSION = 2;

(Same error in src/opencl_kde_depth_packet_processor.cpp.)

Root cause

cl_ext.h (Khronos OpenCL headers ≥ 3.0.16) now defines:

#define CL_ICDL_VERSION  2

This macro expands the variable name in the local declaration
const int CL_ICDL_VERSION = 2;, turning it into const int 2 = 2;, which
is a hard compile error.

Affected files:

• src/opencl_depth_packet_processor.cpp (line ~254)
• src/opencl_kde_depth_packet_processor.cpp (line ~262)

Fix

Undefine the macro immediately before the local variable declaration:

// Before
const int CL_ICDL_VERSION = 2;

// After
#ifdef CL_ICDL_VERSION
#undef CL_ICDL_VERSION
#endif
const int CL_ICDL_VERSION = 2;

---

Steps to reproduce

git clone https://github.com/OpenKinect/libfreenect2
cd libfreenect2
mkdir build && cd build
cmake ..         # → Error 1 (CMake 4.x)
# After manually bumping cmake_minimum_required to 3.5:
make -j$(nproc)  # → Error 2 (CL_ICDL_VERSION)

---

Patch

See the accompanying pull request for a minimal two-commit fix.