fix: Recognize AppleClang to enable recommended compiler flags on macOS

[azeey]

🦟 Bug fix

Summary

While working on gazebosim/gz-physics#946, I noticed that I wasn't getting any compiler warnings locally (on macOS), but they showed up on CI. It turns out the flags used for gcc on Linux and the ones used for clang on macOS are very different. Examples from build.ninja:

macOS

FLAGS = -stdlib=libc++ -O2 -g -DNDEBUG -std=c++17 -arch arm64 -fPIC

But on Linux

FLAGS = -O2 -g -DNDEBUG  -Wall -Wextra -Wno-long-long -Wno-unused-value -Wfloat-equal -Wshadow -Winit-self -Wswitch-default -Wmissing-include-dirs -pedantic  -std=c++17 -fPIC -fvisibility=hidden

Other than the macOS specific flags like -stdlib and -arch, the flags should have been equivalent where possible. The issue was that the compiler ID for Apple's Clang is 'AppleClang', which was not recognized by the scripts. This change ensures that AppleClang is treated similarly to standard Clang, applying the recommended warning flags.

With the new flags, gz-physics emitts additional warnings related to TYPED_TEST_SUITE for the code like the following tests in test/common:

template <class T>
class SimulationFeaturesShapeFeaturesTest :
  public SimulationFeaturesTest<T>{};
using SimulationFeaturesShapeFeaturesTestTypes =
  ::testing::Types<FeaturesShapeFeatures>;
TYPED_TEST_SUITE(SimulationFeaturesShapeFeaturesTest,
                 SimulationFeaturesShapeFeaturesTestTypes);

and the error message is

/Users/addisuzt/ws/rotary/src/gz-physics/test/common_test/simulation_features.cc:989:58: warning: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Wvariadic-macro-arguments-omitted]
  989 |                  SimulationFeaturesShapeFeaturesTestTypes);
      |                                                          ^
/Users/addisuzt/ws/rotary/src/gz-physics/test/gtest_vendor/include/gtest/gtest-typed-test.h:191:9: note: macro 'TYPED_TEST_SUITE' defined here
  191 | #define TYPED_TEST_SUITE(CaseName, Types, ...)                          \
      |         ^

dd5ab89 suppresses this warning to match gcc on Linux.

Edit: I've marked this as draft because this causes several warnings in other Gazebo libraries. Once those are all addressed, I'll mark this ready for review.

Checklist

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