[CMake] Why do jom/nmake generators require cl to build with mingw?

[John R. Cary]

I was trying to configure a project to use jom or name with the
mingw compilers.

I was configuring with

cmake \
   
-DCMAKE_INSTALL_PREFIX:PATH=C:/winsame/volatile-mingw/txphysics-r1504-ser \
   -DCMAKE_BUILD_TYPE:STRING=RELEASE \
   -DCMAKE_VERBOSE_MAKEFILE:BOOL=TRUE \
   -DCMAKE_INSTALL_ALWAYS:BOOL=TRUE \
   -DCMAKE_COLOR_MAKEFILE:BOOL=FALSE \
   -G 'NMake Makefiles' \
   -DCMAKE_C_COMPILER:FILEPATH='mingw32-gcc' \
   -DCMAKE_CXX_COMPILER:FILEPATH='mingw32-g++' \
   -DCMAKE_Fortran_COMPILER:FILEPATH='mingw32-gfortran' \
   C:/cygwin/home/user/vorpalall-mg/txphysics

but got the warning,

CMake Warning at CMakeLists.txt:10 (project):
   To use the NMake generator, cmake must be run from a shell that can 
use the
   compiler cl from the command line.  This environment does not contain
   INCLUDE, LIB, or LIBPATH, and these must be set for the cl compiler to
   work.

and ultimately the error,

CMake Error at C:/Program Files/CMake 
2.8/share/cmake-2.8/Modules/CMakeRCInformation.cmake:22 
(GET_FILENAME_COMPONENT):
   get_filename_component called with incorrect number of arguments
Call Stack (most recent call first):
   C:/Program Files/CMake 
2.8/share/cmake-2.8/Modules/Platform/Windows-GNU.cmake:60 (enable_language)
   C:/Program Files/CMake 
2.8/share/cmake-2.8/Modules/Platform/Windows-GNU-C.cmake:1 (include)
   C:/Program Files/CMake 
2.8/share/cmake-2.8/Modules/CMakeCInformation.cmake:56 (INCLUDE)
   CMakeLists.txt:2 (PROJECT)

So I did put cl in my path, and now it configures, but this seems strange.
Is it necessary to still have cl when using mingw?

Thx....John Cary