RHEL/SUSE/Cray platforms and others using lib64 were broken when building zlib. This fixes that. Relevant to discussion in #121 (but independent of that)
use latest Zlib 2.1.5
only use found Zlib if adler32_combine is present
given above, make default to build Zlib OFF
lint: use SC_IS_TOP_LEVEL to be more explicit in intent
partly revert #122 relevant to CMake packaging to use the default GNUInstallDirs that other CMake packages expect, particularly for RHEL/SUSE/Cray systems