Check if a macro has already been defined with AC_DEFINE

[Quaquaraqua]

Hi there,
how can we check if a macro has been previously defined with AC_DEFINE ?

For instance, something like the following piece of code:
 
AX_OPENMP dnl it sets AC_DEFINE(HAVE_OPENMP,1) in case of success
...
m4_ifdef([[HAVE_OPENMP]], dnl how to check if HAVE_OPENMP has been defined?
    [ ... do something ... ],
    [ ... do something else ... ]
)

I'm aware that, as workaround, AX_OPENMP can take two optional arguments, 
ACTION-IF-FOUND and ACTION-IF-NOT-FOUND but I am
restricted to use them atm. 

Thanks,

-- 
Quaquaraqua <address@hidden>