[cmake] remove custom CheckCXXCompileFlag.cmake macro

[linev]

ROOT was maintain own copy of CheckCXXCompileFlag.cmake from year 2011
Meanwhile cmake 3.20 and higher provides much better version which covers many other compilers with their error output. So there is no reason for maintain our own version

One more reason to remove it - cmake 4.4 produces warning about usage of backslashes in the REGEX.

Some more info from Claude:

for a CMake ≥ 3.20 baseline, the stock module should be functionally equivalent (or better) than ROOT's bundled copy — the original reason for ROOT carrying its own version has effectively been absorbed upstream.

Here's the comparison:

ROOT's custom CheckCXXCompilerFlag.cmake (the version I found) is a 2011-era fork of the old Kitware module. Its whole reason for existing was an extended FAIL_REGEX list to catch compilers that print a warning and exit 0 on a bad flag instead of failing outright — with patterns for GNU, Clang, MSVC, Intel, HP, SunPro, XL/AIX, and PGI.

Stock CMake's module now delegates its regex list to a shared helper, CMakeCheckCompilerFlagCommonPatterns.cmake, included via CHECK_COMPILER_FLAG_COMMON_PATTERNS. That file's pattern set is actually broader than ROOT's own:

[guitargeek]

LGTM, thanks!