CMakeLists: rename version to VERSION to avoid shadowing C++20 <version>

[heitbaum]

Problem

The file named version in the repository root contains the plain-text project version string ("1.1.0") and is read by CMakeLists.txt at configure time:

file(READ version PROJECT_VERSION)

When rpi_ws281x is used as a CMake subdirectory of a C++20 project, the subdirectory's source path is typically added to the compiler include search path so that the library headers (ws2811.h, rpihw.h, etc.) can be found with angle-bracket includes. On any compiler that implements C++20, the <version> feature-test header is part of the standard library. When a translation unit (directly, or through a transitively included header) does:

#include <version>

the preprocessor searches the include path in order. If the rpi_ws281x source directory appears on that path before the toolchain's system include directory, the compiler opens the plain-text version file instead of the C++20 standard library header. Attempting to parse "1.1.0" as C++ source is a hard error:

error: stray '.' in program

Fix

Rename version to VERSION. On Linux (and all other case-sensitive filesystems) VERSION and version are distinct names; the compiler looking for <version> will no longer find the version data file.

Update the single file(READ ...) call in CMakeLists.txt accordingly. No other file in this repository references the plain name version as a path: version.h.in and version.py are separate files with different names and are unaffected.

Impact

Behaviour is identical for all existing build methods (CMake, SCons). The only observable change is the filename on disk; the content and the PROJECT_VERSION variable it populates are unchanged.