Meson doesn't escape newlines when generating build.ninja

[LAK132]

Describe the bug
cmake custom command strings with newlines aren't escaped correctly when generating ninja build scripts.

while attempting to build llvm from source with meson:

cmake = import('cmake')

if get_option('buildtype') == 'debug'
	llvm_build_type = 'Debug'
elif get_option('buildtype') == 'release'
	llvm_build_type = 'Release'
elif get_option('buildtype') == 'debugoptimized'
	llvm_build_type = 'RelWithDebInfo'
elif get_option('buildtype') == 'minsize'
	llvm_build_type = 'MinSizeRel'
else
	error('unsupported build type')
endif

llvm_parallel_jobs = 8

llvm_opts = cmake.subproject_options()

llvm_opts.add_cmake_defines({
	'LLVM_PARALLEL_COMPILE_JOBS': llvm_parallel_jobs,
	'LLVM_PARALLEL_LINK_JOBS': llvm_parallel_jobs,
	'LLVM_TARGETS_TO_BUILD': 'host',
	'LLVM_ENABLE_PROJECTS': 'llvm;clang;compiler-rt',
	'CMAKE_BUILD_TYPE': llvm_build_type,
})

llvm_subprj = cmake.subproject('llvm', options: llvm_opts)

it appears that meson is tripping up on the following in AddLLVM.cmake:

    add_custom_command(OUTPUT ${native_export_file}
      COMMAND "${Python3_EXECUTABLE}" -c "import sys;print(''.join(['EXPORTS\\n']+sys.stdin.readlines(),))"
        < ${export_file} > ${native_export_file}
      DEPENDS ${export_file}
      VERBATIM
      COMMENT "Creating export file for ${target_name}")

it ends up putting a literal newline into the build.ninja instead of an escaped newline:

#############################################
# Custom command for llvm\tools\lto\LTO.def

build llvm\tools\lto\LTO.def | ${cmake_ninja_workdir}llvm\tools\lto\LTO.def: CUSTOM_COMMAND D$:\Repos\lak-clean\subprojects\llvm\llvm\tools\lto\lto.exports
  COMMAND = cmd.exe /C "cd /D D:\Repos\lak-clean\build\subprojects\llvm\__CMake_build\llvm\tools\lto && C:\Users\lakri\AppData\Local\Programs\Python\Python310\python.exe -c "import sys" "print(''.join(['EXPORTS
']+sys.stdin.readlines(),))" < D:/Repos/lak-clean/subprojects/llvm/llvm/tools/lto/lto.exports > LTO.def"
  DESC = Creating export file for LTO
  restat = 1

which results in an error:

| CMake Error:
| Running

| 'C:/PROGRA~1/Meson/ninja.EXE' '-C' 'D:/Repos/lak-clean/build/subprojects/llvm/__CMake_build' '-t' 'recompact'

| failed with:

| ninja: error: build.ninja:41934: lexing error

Expected behavior
meson should escape newlines when generating ninja build script

system parameters

• native build
• windows 10
• python 3.10
• meson 0.63.2
• ninja 1.11.0