So now, after make, I get the following:
[localhost:~/althea] mulvihiw% make
c++ -Wall -ftemplate-depth-30 -g -lssl -lcrypto -o althea main.cpp -DPIXDIR=\"/usr/local/share/althea\" support.o callbacks.o new_message_window.o about_window.o main_window.o Message.o Folder.o Server.o Configuration.o Althea.o ABEntry.o Addressbook.o load_config.o SendMail.o tcpUtilities.o imap.o basicNetwork.o SelectFolder.o print_dialog.o helpDialog.o smtp.o save_config.o configwindow.o registerwindow.o register.o crypto.o promptwindow.o progressbar.o tolower.o MIME.o `gtk-config --cflags` -DPACKAGE=\"althea\" -DLOCALEDIR=\"/usr/share/locale\" `gtk-config --libs`

/usr/bin/ld: warning can't open dynamic library: /usr/X11R6/lib/libXext.6.dylib (checking for undefined symbols may be affected) (No such file or directory, errno = 2)

/usr/bin/ld: warning can't open dynamic library: /usr/X11R6/lib/libX11.6.dylib (checking for undefined symbols may be affected) (No such file or directory, errno = 2)

But it does compile. But actually running the programs gives you:
[localhost:~/althea] mulvihiw% ./althea
dyld: ./althea can't open library: /usr/X11R6/lib/libXext.6.dylib (No such file or directory, errno = 2)
[localhost:~/althea] mulvihiw%

I'm guessing that I don't have all the necessary libraries installed, but which package should give them to me? I would have figured that libXext and libX11 would have been installed by default with X-Windows...