b2 uses the wrong slashes for gcc under windows in .rsp files

[kfbBjoern]

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Environment and version details

• Operating System+version: Windows 11
• Compiler+version: mingw gcc-12.1.0
• Shell: cmd, poweshell
• B2 Version: Output of b2 -v and b2 --version B2 5.2.1 (OS=NT, jobs=48)
• B2 Configuration: Output of b2 --debug-configuration in your project.

...
D:/Dependencies/mingw64-12.1.0/bin/../lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lmkl_rt: No such file or directory
collect2.exe: error: ld returned 1 exit status

    "../../../Dependencies/mingw64-12.1.0/bin/g++.exe" @"Library/MultibodySystem/gcc-12.1.0/debug/libCaesMultibodySystem.dll.rsp"
....

When the linking command is to large a .rsp file is written. The slashes for the library path are bakshlahes and not recogneized by the linker

Steps to reproduce the issue

Actual behavior summary

The .rsp files look like

-L"..\..\..\Third_Party\MKL-2021.3b\Library\MKL\gcc-12.1.0\release\address-model-64"  -Wl,--out-implib,"Library/MultibodySystem/gcc-12.1.0/debug/libCaesMultibodySystem.dll.a" -o "Library/MultibodySystem/gcc-12.1.0/debug/libCaesMultibodySystem.dll"  -shared -Wl,--start-group "Library/MultibodySystem/gcc-12.1.0/debug/AccelerationsProvider.o" "Library/MultibodySystem/gcc-12.1.0/debug/DummyIntegrator.o" "Library/MultibodySystem/gcc-12.1.0/debug/FilesFunctions.o" "Library/MultibodySystem/gcc-12.1.0/debug/IntegratorsSetter.o" 
..
"Library/MultibodySystem/gcc-12.1.0/debug/Vectors3.o" "Library/MultibodySystem/gcc-12.1.0/debug/ZXZAnglesToEulerParamsSignal.o" "Library/MultibodySystem/gcc-12.1.0/debug/MultibodySystem_res.o"  -Wl,-Bstatic  -Wl,-Bdynamic -lmkl_rt -Wl,--end-group -g 

**The project is blocked since we have to use the newest msvc and the gcc in version 12.1 for WIN11 **

Expected behavior summary

I think the .rsp files should look l ike this as this can be linked without errors

-L"../../../Third_Party/MKL-2021.3b/Library/MKL/gcc-12.1.0\release\address-model-64"  -Wl,--out-implib,"Library/MultibodySystem/gcc-12.1.0/debug/libCaesMultibodySystem.dll.a" -o "Library/MultibodySystem/gcc-12.1.0/debug/libCaesMultibodySystem.dll"  -shared -Wl,--start-group "Library/MultibodySystem/gcc-12.1.0/debug/AccelerationsProvider.o" "Library/MultibodySystem/gcc-12.1.0/debug/DummyIntegrator.o" "Library/MultibodySystem/gcc-12.1.0/debug/FilesFunctions.o" "Library/MultibodySystem/gcc-12.1.0/debug/IntegratorsSetter.o" 
..
"Library/MultibodySystem/gcc-12.1.0/debug/Vectors3.o" "Library/MultibodySystem/gcc-12.1.0/debug/ZXZAnglesToEulerParamsSignal.o" "Library/MultibodySystem/gcc-12.1.0/debug/MultibodySystem_res.o"  -Wl,-Bstatic  -Wl,-Bdynamic -lmkl_rt -Wl,--end-group -g