CMake: Enable external ZLIB from CMake variable ZLIB_ROOT

[lordjaxom]

We build Blosc from scratch using CMake's ExternalProject and would like to use an external ZLIB.

Since our goal is to have as many projects build closely to CMake standards, we would like to provide -DZLIB_ROOT when configuring Blosc and have find_package(ZLIB) pick up $ZLIB_ROOT/.../ZLIBConfig.cmake. Currently this is prevented because Blosc's CMakeLists.txt replaces ZLIB_ROOT with $ENV{ZLIB_ROOT}. Sadly we can't set environment variables with ExternalProject's CMake build feature. The attached patch only replaces ZLIB_ROOT if it is not already defined.

blosc-1.21.3-z1.patch