Maintaining building external/coinmp on Windows

[Stephan Bergmann]

My attempt at <https://gerrit.libreoffice.org/c/core/+/140057/1> 
"external/coinmp: Update to CoinMP 1.8.4" failed on Windows at 
<https://ci.libreoffice.org/job/gerrit_windows/136050/> with

> libCbcSolver.lib(CbcSolverHeuristics.obj) : error LNK2019: unresolved external symbol "public: __thiscall CbcHeuristicDW::CbcHeuristicDW(class CbcModel &,int)" (??0CbcHeuristicDW@@QAE at AAVCbcModel@@H at Z) referenced in function "int __cdecl doHeuristics(class CbcModel *,int,class CbcOrClpParam *,int,int,int)" (?doHeuristics@@YAHPAVCbcModel@@HPAVCbcOrClpParam@@HHH at Z) [C:\cygwin\home\tdf\lode\jenkins\workspace\gerrit_windows at 2\workdir\UnpackedTarball\coinmp\CoinMP\MSVisualStudio\v9\CoinMP\CoinMP.vcxproj]
> libCbcSolver.lib(CbcSolverHeuristics.obj) : error LNK2019: unresolved external symbol "public: virtual __thiscall CbcHeuristicDW::~CbcHeuristicDW(void)" (??1CbcHeuristicDW@@UAE at XZ) referenced in function "int __cdecl doHeuristics(class CbcModel *,int,class CbcOrClpParam *,int,int,int)" (?doHeuristics@@YAHPAVCbcModel@@HPAVCbcOrClpParam@@HHH at Z) [C:\cygwin\home\tdf\lode\jenkins\workspace\gerrit_windows at 2\workdir\UnpackedTarball\coinmp\CoinMP\MSVisualStudio\v9\CoinMP\CoinMP.vcxproj]
> libCgl.lib(CglGMI.obj) : error LNK2019: unresolved external symbol "private: bool __thiscall CoinRational::nearestRational_(double,double,long)" (?nearestRational_ at CoinRational@@AAE_NNNJ at Z) referenced in function "public: __thiscall CoinRational::CoinRational(double,double,long)" (??0CoinRational@@QAE at NNJ@Z) [C:\cygwin\home\tdf\lode\jenkins\workspace\gerrit_windows at 2\workdir\UnpackedTarball\coinmp\CoinMP\MSVisualStudio\v9\CoinMP\CoinMP.vcxproj]
> C:\cygwin\home\tdf\lode\jenkins\workspace\gerrit_windows at 2\workdir\UnpackedTarball\coinmp\CoinMP\MSVisualStudio\v9\Debug\CoinMP.dll : fatal error LNK1120: 3 unresolved externals [C:\cygwin\home\tdf\lode\jenkins\workspace\gerrit_windows at 2\workdir\UnpackedTarball\coinmp\CoinMP\MSVisualStudio\v9\CoinMP\CoinMP.vcxproj]

presumably because "There's apparently a new ClpSolver library now" 
needs to be reflected in our external/coinmp/windows.build.patch.1.

But I have no idea what to do with that file, and neither 
external/coinmp/README nor 
<https://git.libreoffice.org/core/+/dbe6e76332c4eda87aa2d2168dd14c274a4930d4%5E!/> 
"Add CoinMP external as an alternative to lpsolve" (which introduced the 
file) give a hint.  :(