From what I recall reading recently you'd probably have to install both PostgreSQL and PostGIS from source in that case.
This isn't particularly difficult but you need to make sure you have all the appropriate development packages installed. Often times this will be trial and error (run ./configure, see what it complains about, install the package, wash, rinse, repeat).
if you go this route, I highly recommend running ./configure --help and reading through all the options. The PostgreSQL source is very configurable and this is easy. From there, though, PostGIS should be a piece of cake.