[lldb][cmake] Error out when building debugserver with CMake 4

CMake 4 no longer sets the `CMAKE_OSX_SYSROOT` variable by default. If
you've updated to CMake 4 on macOS (e.g. with brew) and try building LLDB with
CMake/ninja, this will yield an error when building debugserver that
clang is unable to run since it tries to compile files that don't exist.

These files are supposed to be generated by the `mig` process. `mig`
needs the `CMAKE_OSX_SYSROOT` variable in order to work and without it,
it silently fails to generate the files that later on need to be
compiled.

This commit will fatal error out of config when building debugserver
without having set CMAKE_OSX_SYSROOT.

Author: chelcassanova

lldb/tools/debugserver/source/CMakeLists.txt

@@ -154,6 +154,20 @@ endif()
 
 add_definitions(-DLLDB_USE_OS_LOG)
 
+if(NOT CMAKE_OSX_SYSROOT)
+  execute_process(COMMAND xcodebuild -version -sdk macosx Path
+    OUTPUT_VARIABLE SDKROOT
+    ERROR_QUIET
+    OUTPUT_STRIP_TRAILING_WHITESPACE)
+
+  if(NOT EXISTS ${SDKROOT})
+    message(FATAL_ERROR "Unable to obtain macOS SDK root, debugserver cannot be built.")
+  endif()
+
+  message(STATUS "Using macOS SDK root: ${SDKROOT}")
+  set(CMAKE_OSX_SYSROOT ${SDKROOT})
+endif()
+
 if(${CMAKE_OSX_SYSROOT} MATCHES ".Internal.sdk$")
   message(STATUS "LLDB debugserver energy support is enabled")
   add_definitions(-DLLDB_ENERGY)