I know, I run postgis+mapnik on OSX. It is quite PITA to build, postgres clients usually link against wrong (system-supplied & ancient) libpq[1], everything links against wrong (system-supplied & ancient) zlib, half the things link against wrong (system-supplied & ancient; see the pattern?) sqlite, etc. Yes, it is problem with the build scripts, but I don't have an ambition to fix all the packages, and Linux packages do not have this problem. Actually, they have usable and maintained binaries just a single yum command away, so you are up and running in no time, compared to solving build env problems in OSX.

[1] - /usr/lib/libpq.dylib is especially annoying offender. Most dylibs are at least fat ones, but this one is x86_64 only. That means, when you want to build a fat binary of some libpq client, and it is being linked against system one, the build will fail. That includes building i386/fat version of postgresql itself.