[CMake] ADD_DEPENDS Fortran only

[Karl Merkley]

So, I have this little mixed C++/Fortran project that I have been 
working on . . .

> PROJECT(add_fortran CXX Fortran)
> ADD_EXECUTABLE(add_fortran main.cpp testf1.F testf2.f)
>
> # Must specify which linker to use for a multiple language project
> SET_TARGET_PROPERTIES(add_fortran PROPERTIES LINKER_LANGUAGE CXX)
>
> # Using g77 need to find and link with libg2c
> FIND_LIBRARY(G2C NAMES g2c PATHS /opt/local/lib)
> TARGET_LINK_LIBRARIES(add_fortran ${G2C})

Now, I want to add a define to the Fortran only.   It looks like a

ADD_DEFINITIONS(-DTEST_DEFINE)

will be added to the C++ compile line also.  How do I define this only 
for the Fortran?  Is the best way to just do something like

SET(CMAKE_Fortran_FLAGS "-DTEST_DEFINE")

   Karl