Properly respect CFLAGS, LDFLAGS, etc.
When building a port with base system make, variables like CC and CFLAGS
need to be defined using '=' in the makefile. With '+=' values from
make.conf can be picked up. Normally the ports system values for these
variables are passed to a configure script which generates a makefile with
them. If there's no configure script the makefile has to be patched to use
the right values.