Finding libraries to link with and autoconf resources?

[Garrett Cooper]

Hello,
	This is my first attempt at porting an application, so please bear with me.
	I'm trying to port tvtime from Linux to FreeBSD and it's failing during 
the configure stage (claims it can't find -lpng when running a link 
test). I know I should probably use the Linux libpng library but I'm 
trying to make the application as native as possible with FreeBSD.
	Anyhow though, I was wondering what directories I should typically 
search when using -L for searching for the png libraries, for instance 
(/usr/lib.. ?).
	Also, are there any suggestions for creating / modifying autoconf 
generated files?
Thanks,
-Garrett