Many Compile Warnings while Installing MacPorts
I encounter this most when I see compile flags including -Wall (warn all) which a great deal of open source devs tend to use.  Warnings are not always bad or even worrisome.  

If you're paranoid, take the time to go through and investigate each of the warnings but before you do that try compiling some code of your own with the warn all flag set for gcc and see how many warnings you get.  They are quite common.