Fix macOS build with Homebrew LLVM

On macOS with Homebrew LLVM, explicitly adding LLVM_INCLUDE_DIRS breaks
libc++ header search order (include_next can't find libc++'s wrapper
headers). The Homebrew clang compiler already knows where its LLVM/Clang
headers are located, so we don't need to add them explicitly.

On Linux and other platforms, we still add the include paths explicitly
since system LLVM packages don't interfere with the stdlib.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: artemdinaburg

lib/CMakeLists.txt

@@ -133,13 +133,16 @@ target_include_directories("${PROJECT_NAME}"
     $<INSTALL_INTERFACE:include>
 )
 
-# Use SYSTEM to avoid interfering with stdlib header search order on macOS
-# (Homebrew LLVM bundles its own libc++ and -I paths break include_next)
-target_include_directories("${PROJECT_NAME}" SYSTEM
-  PUBLIC
-    ${LLVM_INCLUDE_DIRS}
-    ${CLANG_INCLUDE_DIRS}
-)
+# On macOS with Homebrew LLVM, the compiler already knows where its headers are.
+# Adding explicit -I/-isystem paths breaks libc++ header search order (#include_next fails).
+# On Linux/other platforms, we need to add the paths explicitly.
+if(NOT APPLE)
+  target_include_directories("${PROJECT_NAME}"
+    PUBLIC
+      ${LLVM_INCLUDE_DIRS}
+      ${CLANG_INCLUDE_DIRS}
+  )
+endif()
 
 if(RELLIC_ENABLE_INSTALL)
   include(GNUInstallDirs)