[CMake] help with cmake fortran envar

[luxInteg]

Greetings,

I am learning cmake.


I have a fortran only project with some  source files with f77 routines  to 
compile,  My CMakeLists.txt in the projects base directory   has the following 
lines

set(CMAKE_Fortran_COMPILER  gfortran)
ENABLE_LANGUAGE(Fortran)


########  but running cmake ends like so:-

-- Checking whether /usr/bin/gfortran supports Fortran 90
-- Checking whether /usr/bin/gfortran supports Fortran 90 -- yes
CMake Error: Error required internal CMake variable not set, cmake may be not 
be built correctly.
Missing variable is:
CMAKE_FORTRAN_COMPILER_ENV_VAR
CMake Error: Error required internal CMake variable not set, cmake may be not 
be built correctly.
Missing variable is:
CMAKE_FORTRAN_COMPILER
CMake Error: Could not find cmake module file:
$SOURCES/build/CMakeFiles/CMakeFORTRANCompiler.cmake
CMake Error: Could not find cmake module file:CMakeFORTRANInformation.cmake
CMake Error: CMAKE_FORTRAN_COMPILER not set, after EnableLanguage
-- Configuring incomplete, errors occurred!




#########

I have set 
FC="gfortran"       and 
FFLAGS="flags/I/use/to/compile/stuff" 
as envars so I am perplexed what the required envar is.

advice would be appreciated

sincerely
luxInteg