Instead of defining a variable that is almost always based on CONFIGURE_ENV, just use CONFIGURE_ENV directly. This also matches the behavior of other ports that do not use autotools (so most ports can just worry about CONFIGURE_ENV). Additionally, the fact that we do not use ?= means we do not have problems if another file in Uses/ needs to set CONFIGURE_ENV (with CMAKE_ENV, the order of the arguments to USES would matter).

All mentioned ports have been tested with the change and build fine on 9.3-i386.