package: enable _LIBCPP_REMOVE_TRANSITIVE_INCLUDES for all libc++ builds.

Which makes libc++ since version 16 not use transitive includes for
backwards compatibility, both for faster build times and to detect if
some code is missing a transitive include (which could blow up for the
user even w/o this macro defined, as compiling with -std=c++23 also
removes quite a lot such includes).

For this repository this affects solely macOS on the CI, but right now
it uses a VM with older libc++, which means it does nothing, but it's
done there anyway to future-proof.

package/ci/circleci.yml

@@ -394,7 +394,11 @@ jobs:
   macos-gl:
     executor: xcode-14_3
     environment:
-      CMAKE_CXX_FLAGS: --coverage
+      # Make libc++ remove transitive includes, both for faster build times and
+      # to detect if we're missing a transitive include. Works with libc++ 16+,
+      # which is used by Xcode 15 (i.e., will get used on the next CircleCI
+      # executor update).
+      CMAKE_CXX_FLAGS: --coverage -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES
       PLATFORM_GL_API: CGL
     steps:
     - install-base-macos:
@@ -410,7 +414,11 @@ jobs:
   macos-gles3:
     executor: xcode-14_3
     environment:
-      CMAKE_CXX_FLAGS: --coverage
+      # Make libc++ remove transitive includes, both for faster build times and
+      # to detect if we're missing a transitive include. Works with libc++ 16+,
+      # which is used by Xcode 15 (i.e., will get used on the next CircleCI
+      # executor update).
+      CMAKE_CXX_FLAGS: --coverage -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES
       PLATFORM_GL_API: EGL
       # STUPID yml interprets unquoted OFF as a boolean
       TARGET_GLES2: "OFF"
@@ -432,7 +440,11 @@ jobs:
     environment:
       # STUPID yml interprets unquoted ON as a boolean
       BUILD_STATIC: "ON"
-      CMAKE_CXX_FLAGS: --coverage
+      # Make libc++ remove transitive includes, both for faster build times and
+      # to detect if we're missing a transitive include. Works with libc++ 16+,
+      # which is used by Xcode 15 (i.e., will get used on the next CircleCI
+      # executor update).
+      CMAKE_CXX_FLAGS: --coverage -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES
       PLATFORM_GL_API: CGL
     steps:
     - install-base-macos: