Enable use of std::format for AppleClang / Xcode 16

Specified the minimum apple-clang version to support `std::format` to 16
for _connanfile.py_.  Xcode 16 does include the `<format>` header but
does not seem to have the __cpp_lib_format compatibility macro defined
so a similar override was needed as we had for Clang 17.

To be able to use `std::format` with Xcode 16 I had to pass
`-DMP_UNITS_API_STD_FORMAT=AUTO` when calling `cmake`.  Is this expected
or is this a sign I missed something in my changes to _CMakeLists.txt_?

conanfile.py

@@ -99,7 +99,7 @@ class MPUnitsConan(ConanFile):
                 "compiler": {
                     "gcc": "13",
                     "clang": "17",
-                    "apple-clang": "",
+                    "apple-clang": "16",
                     "msvc": "194",
                 },
             },

src/CMakeLists.txt

@@ -53,13 +53,15 @@ check_cxx_feature_supported(__cpp_explicit_this_parameter ${projectPrefix}EXPLIC
 
 # libc++ has a basic supports for std::format but does not set __cpp_lib_format
 # https://github.com/llvm/llvm-project/issues/77773
-if(NOT ${projectPrefix}LIB_FORMAT_SUPPORTED
-   AND CMAKE_CXX_COMPILER_ID STREQUAL "Clang"
-   AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL "17"
-   AND ${projectPrefix}LIBCXX
-)
+if(NOT ${projectPrefix}LIB_FORMAT_SUPPORTED AND ${projectPrefix}LIBCXX)
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL "17")
         message(STATUS "Clang 17+ with libc++ detected, overriding `std::format` support")
         set(${projectPrefix}LIB_FORMAT_SUPPORTED ON)
+    endif()
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL "16")
+        message(STATUS "AppleClang 16+ with libc++ detected, overriding `std::format` support")
+        set(${projectPrefix}LIB_FORMAT_SUPPORTED ON)
+    endif()
 endif()
 
 # clang++-18 supports explicit `this` parameter