Based on the bug report filed here, Gnash will not compile with the latest giflib. I assume that means that if I use an older giflib, it'll compile just fine. I'm not sure how to do this, though - my package manager (I'm on Manjaro, and have access to AUR) doesn't seem to help here, and I'm not sure what I would have to modify or do in such a case anyway.
Basically, after I get the code for Gnash from Savannah, what would I need to do/change to make it compile using an older giflib?
If you haven't already seen it, as Raydel notes there has been a reply to your request for a workaround on the bug report itself.
This is certainly the quicker solution, though there is nothing wrong with having two versions (or just the older, working) version of the GifLib library on your system.
To do that, you can download GifLib v4.2.3 here, a version older than the change that breaks gnash compilation.
Either place it in a different directory (you probably have v5.1 in /usr/local/lib?) and change the include/linker paths or replace v5.1 files in-place and you shouldn't need to change your compilation steps at all.
Trying the same steps myself, I have been unable to replicate your error.
Excluding sudo apt-get install-ing numerous packages (which aren't the cause of your troubles - and at every step I was told exactly what was missing) these were my steps on a fresh installation:
$ git clone git://git.sv.gnu.org/gnash.git
$ curl http://sourceforge.net/projects/giflib/files/giflib-4.x/giflib-4.2.3.tar.bz2/download
$ tar xvjf giflib-4.2.3.tar.bz2
$ cd giflib-4.2.3
$ ./configure
$ sudo make
$ sudo make install
$ cd ../gnash
$ ./autogen.sh
$ ./configure.ac
$ sudo make
$ sudo make install
I recommend you begin afresh, since these steps took care of everything for me - without any manual moving of files, specifying compiler/linker options or editing of makefiles which seems to be causing you problems.