Cannot build Python SDK on RHEL 8

The Python SDK currently tries to statically link the c++ runtime. Apparently that is not something supported on RHEL8 (reference here and here):

Static Linking with the C/C++ Runtime

• Static linking with the C/C++ runtime is not supported. This includes linking with any files that are part of the glibc-static or libstdc++-static packages. You may choose to link statically, but the resulting application binaries may fail to operate if any package in the installation is changed.

If we do not set COUCHBASE_CXX_CLIENT_STATIC_STDLIB to ON we can successfully build on a RHEL 8 system. So, lets allows COUCHBASE_CXX_CLIENT_STATIC_STDLIB to be set to OFF in order to skip statically linking with stdc++ libs. Note, currently this only impacts linux systems (specifically RHEL). See couchbase++ CMakeLists.