[GENERAL] Using different GCC, CFLAGS, CCFLAGS and CPPFLAGS to compile Postgresand PostGIS?

I am wondering about this question for a while with no definite answer to it, can someone explain me in detail to clear me out on the following question :

What can go wrong or is it acceptable if I build Postgres from source with let's say GCC 4.x.y and some specific CFLAGS, CPPFLAGS and CCFLAGS and when I try to install an extension for Postgres such as PostGIS using a different version of GCC lets say GCC 4.x.z with varying CFLAGS, CCFLAGS and CPPFLAGS and if I follow the same process for other non-contrib extensions? I want to know the theory behind this and it would serve me a great help to understand the systems aspect of the process as well.

Thanks