build(macos): use consistent MACOSX_DEPLOYMENT_TARGET (#19430)

Use the same logic for both deps (including LuaJIT, for which setting
this variable is mandatory) and Nvim: either the eponymous environment
variable if set, or the current software version if not.

Removes annoying warnings when building locally on macOS.

cmake.deps/CMakeLists.txt

@@ -132,6 +132,18 @@ if(CMAKE_OSX_ARCHITECTURES)
   endforeach()
 endif()
 
+# If the macOS deployment target is not set manually (via $MACOSX_DEPLOYMENT_TARGET),
+# fall back to local system version. Needs to be done here and in top-level CMakeLists.txt.
+if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+  if(NOT CMAKE_OSX_DEPLOYMENT_TARGET)
+    execute_process(COMMAND sw_vers -productVersion
+                    OUTPUT_VARIABLE MACOS_VERSION
+                    OUTPUT_STRIP_TRAILING_WHITESPACE)
+    set(CMAKE_OSX_DEPLOYMENT_TARGET "${MACOS_VERSION}")
+  endif()
+  message("-- Using deployment target ${CMAKE_OSX_DEPLOYMENT_TARGET}")
+endif()
+
 set(HOSTDEPS_INSTALL_DIR "${DEPS_INSTALL_DIR}")
 set(HOSTDEPS_BIN_DIR "${DEPS_BIN_DIR}")
 set(HOSTDEPS_LIB_DIR "${DEPS_LIB_DIR}")