Speeding up compile times

[rockcavera]

Here the difference is quite noticeable according to the C compiler used. See 
the Nim devel compiler build time:

  *  **LLVM/CLANG 16.0.1** : 111.20s
  * **GCC/niXman mingw-builds 12.2.0** : 157.35s
  * **GCC/winlibs 12.2.0** : 416.66s



All cache is deleted before each test run and compilation is always done with 
the same nim.exe resulting from csources_v2.

Commands used to build the compiler:
    
    
    @ECHO OFF
    SET CC=--cc:gcc|clang
    RMDIR /S /Q E:\Nim\nimcache
    COPY /Y 
E:\Nim\bin\nim_csources_86742fb02c6606ab01a532a0085784effb2e753e.exe 
E:\Nim\bin\nim.exe
    call timecmd koch boot -d:release --skipUserCfg --skipParentCfg %CC%
    
    
    Run

Intel i7-4810MQ, Windows 10 64-bit.