Fix erroneous test for XCode 15.1

[asimilon]

With Xcode 15.3 installed:

/usr/bin/c++ --version
Apple clang version 15.0.0 (clang-1500.3.9.4)

CMake reports:

-- The C compiler identification is AppleClang 15.0.0.15000309
-- The CXX compiler identification is AppleClang 15.0.0.15000309

Hence this test does not work as intended:
if( ${CMAKE_CXX_COMPILER_VERSION} VERSION_GREATER_EQUAL "15.0.0" AND ${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS "15.1")

To confirm my suspicions about how the version number is reported by Apple Clang I booted into an old install running Xcode 14.2:

/usr/bin/c++ --version
Apple clang version 14.0.0 (clang-1400.0.29.202)

which CMake reports as:

-- The C compiler identification is AppleClang 14.0.0.14000029
-- The CXX compiler identification is AppleClang 14.0.0.14000029

so the real version number of Clang is in the 4th part, and takes the format for 2 digits for each part, dropping the final part from the longer number reported from --version.

I think this revised test is OK, although I was unable to test against a relevant version of Xcode that needs the linker flags and change of deployment target.

[asimilon]

btw I found this list of Xcode and corresponding Clang versions, which shows:

# Xcode 15.1 (15C65) 
Apple clang version 15.0.0 (clang-1500.1.0.2.5)

so I think that the corresponding CMAKE_CXX_COMPILER_VERSION for this version would be 15.0.0.15000100 and my updated version test should hold, as both the prior Xcode versions are:

# Xcode 15.0 (15A240d)
Apple clang version 15.0.0 (clang-1500.0.40.1)

which should show as 15.0.0.15000040 (assuming there is logic to the Apple Clang version number!)

[jatinchowdhury18]

Thanks for looking into this! The change appears correct to me, but I'd prefer to wait for Paul to have a look before merging.

[baconpaul]

I’m on vaca right now but it also looks correct esp with the crazy cmake versions whic we guessed before Xcode 15.1 even shipped! Let’s merge it.