If you built SDL 1.x from current 1.x branch of SDL’s mercurial it might work (or you might need to adapt your source).
I’m building Exult against SDL 1.x and it still works in El Capitan.
You need to run ./autogen.sh first.
Also depends, if you are making use of the framework, then use xcode to build SDL (through the xcode.project file somewhere).
You probably don’t need the x11 backend. Disable it via ./configure --without-x
But check via ./configure --help whether I gave you the correct command 
Or download and install xqartz and add the path to the xqartz includes.
Or check with brew whether they provide sdl 1.x devel…
Make install should have put the sdl libs somewhere in your prefix. No idea what that is though with homebrew. /usr/local/libs or somewthing like that.
But sdl_mixer is seperate from the sdl sources, you need to download that from its own repo. For that you can get the last release sources (aimed at sdl 1.x) since that didn’t get any relevant fixes, I think.
Great! Happy to be helpful 