[Gmsh] building of gmsh on mingw64

[walter]

Hello,
  I am used to build my application (EmCAD) and gmsh on Msys2/MINGW64 platform.

Today I made a small change to the main CMakeLists.txt file of gmsh-4.6.0-source.
This change fixes the following problem:
  cmake/configure is not able to find the OCC libraries on Msys2/MINGW64 platform even if CASROOT is properly set.

This problem arises because:
 1)  on Msys2/MINGW64 platform the OCC libraries are located in the following folders
     
     $CASROOT/${OCC_SYS_NAME}/gcc/lib  >> dll.a  libraries 
     $CASROOT/${OCC_SYS_NAME}/gcc/bin  >> dll    libraries


 So I made the following small changes:


 line 12179 to 1273
        if(ENABLE_OCC_STATIC)
          set(LIBNAME lib${OCC}.a)
        else()
          set(LIBNAME ${OCC})
        endif()

 is replaced by:        
        if(ENABLE_OCC_STATIC)
	  if(MINGW)
             set(LIBNAME lib${OCC}.dll.a)
	  else()
             set(LIBNAME lib${OCC}.a)
          endif()
        else()
          set(LIBNAME ${OCC})
        endif()


 line 1284 to 1285:        
        find_library(OCC_LIB ${LIBNAME} HINTS ENV CASROOT PATH_SUFFIXES
                     lib ${OCC_SYS_NAME}/lib ${OCC_SYS_NAME}/vc8/lib)

 is replaced by:      
        find_library(OCC_LIB ${LIBNAME} HINTS ENV CASROOT PATH_SUFFIXES lib bin
                     lib ${OCC_SYS_NAME} ${OCC_SYS_NAME}/vc8 ${OCC_SYS_NAME}/gcc )


 line 1306 to 1307:  
        find_library(OCC_CAF_LIB ${OCC} HINTS ENV CASROOT PATH_SUFFIXES
                    lib ${OCC_SYS_NAME}/lib ${OCC_SYS_NAME}/vc8/lib)

 is replaced by:
        find_library(OCC_CAF_LIB ${OCC} HINTS ENV CASROOT PATH_SUFFIXES lib bin
                    lib ${OCC_SYS_NAME} ${OCC_SYS_NAME}/vc8 ${OCC_SYS_NAME}/gcc)


-bin has been added to PATH_SUFFIXES in both calls to find_library.
-${OCC_SYS_NAME}/gcc subfolder explitely added to ${OCC_SYS_NAME}/vc8
-lib was removed from some explicit subfolders because it is already appended by PATH_SUFFIXES

With this change, when the option ENABLE_OCC_STATIC is activated on a Msys2/MINGW64 platform, gmsh is linked with libTKernel.dll.a ...

libTKernel.dll.a is an import library in the microsoft world. 
I think that this kind of library is meant to import the libTKernel.dll library placed in the gcc/bin folder.
I do not know if it make any difference with direct linkage to the dll.


Walter