[Gmsh] building on MSYS2

[walter steffe]

Hello,
  I would like to report that I have just succesfully built gmsh
library (with disabled fltk) on MSYS2 platform with mingw-64 compilers.

To do that it was necessary to change a line (line number 176
within gmsh 4.04) of the main CMakeLists.txt file:

if(APPLE)
  set(GMSH_OS "MacOSX")
elseif(CYGWIN)
.....

now is:

if(APPLE)
  set(GMSH_OS "MacOSX")
elseif(CYGWIN OR MSYS)
.....


Settings used for CYGWIN seems good also for MSYS2.
So I think that you may keep this small change in the future gmsh
versions.

Regards
Walter