How to conditionally compile plug-in source file to only support dhcpcd or dhclient?

[Mu Qiao <qiaomuf gmail com>]

Hi Dan,

Gentoo NM plugin has to read dhcp hostname and clientid from
configuration file of dhcpcd or dhclient.

Currently I just test whether the configuration file of dhcpcd or
dhclient exists to determine which one is used. Could I make use of the
switch "--with-dhcpcd=yes" to enable conditional compilation?
for example:
#ifdef DHCPCD_PATH
//handling dhcpcd configuration file
#else
//handling dhclient configuration file
#endif

If this could be achieved, what should follow "#ifdef"? Should I include
any header file?

-- 
Cheers,
Mu Qiao
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