Uses/cmake.mk: Replace CMAKE_ENV with CONFIGURE_ENV.

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).

Ports which set CMAKE_ENV have been adjusted accordingly. In most cases,
CMAKE_ENV was just replaced with CONFIGURE_ENV, the exceptions being:

• databases/sqliteman: CMAKE_ENV line removed; setting QMAKESPEC there has no effect on the build system.
• devel/freeocl: CMAKE_ENV line removed; FREEOCL_CXX_COMPILER is already retrieved from the CMAKE_CXX_COMPILER variable in the build system.
• graphics/openimageio: CMAKE_ENV line removed; setting Qt variables there has no effect on the build system.

Reviewed by: makc
Differential Revision: https://reviews.freebsd.org/D3403