How can I determine if a function is available?

[Ryan Schmidt]

I would like to improve the php5 port so that both an Apache 1 or 2  
module and the FastCGI binary can be installed at the same time. The  
php build system does not accommodate targeting multiple SAPIs at  
once. Therefore it will be necessary to have port build once for the  
Apache module of choice, and then build again for the FastCGI module.

I would like to make use of the strategy that was briefly employed by  
the zlib port to re-run the configure and build phases, but I need  
some advice, because of the following two commits:

http://trac.macosforge.org/projects/macports/changeset/23238

http://trac.macosforge.org/projects/macports/changeset/23409

In r23238, the command "command" was removed, and a new command  
"command_exec" was introduced, but this change did not make it into  
MacPorts 1.4.0.

In r23409, it was shown how this change will affect portfiles: a  
wrapper which called 'system "[command $cmd]"' was replaced with just  
"command_exec $cmd".

So as I understand it, in MacPorts 1.4.0, I must use a wrapper which  
calls 'system "[command $cmd]"'. But whenever r23238 is merged into a  
released version of MacPorts (be that 1.4.1 or 1.5.0), this code will  
break, and it will then be necessary to use "command_exec $cmd".

Is there a way that I can code my portfile today that will work with  
both current and future MacPorts versions? Is there a way that I can  
detect which of "command" or "command_exec" are available in the  
installed version of MacPorts?

Thanks.