Examine the files /etc/defaults/make.conf and /etc/make.conf. The first contains some default defines - most of which are commented out. To make use of them when you rebuild your system from source, add them to /etc/make.conf.
Keep in mind that anything you add to /etc/make.conf is also used every time you run make, so it is a good idea to set them to something sensible for your system.
A typical user will probably want to copy the CFLAGS and NOPROFILE lines found in /etc/defaults/make.conf to /etc/make.conf and uncomment them. Examine the other definitions (COPTFLAGS, NOPORTDOCS and so on) and decide if they are relevant to you.
Most people have at least the following in their /etc/make.conf:
CFLAGS= -O -pipe NOPROFILE=true