cmake: pass LDSHARED env var to distutils

cmake: pass LDSHARED env var to distutils

otherwise, the default gcc will be used, and the $CMAKE_C_COMPILER
passed to the outer CMakeLists.txt won't kick in. moreover, if the
building script (ceph.spec for instance) could set the $PATH, and
expect that the CMakeLists.txt will use the toolchain executables
in the $PATH to build Ceph, distutils will continue using the default
$CC for linking the python bindings, on UNIX it will be gcc in the
new shell's $PATH, because we are using `install(CODE "... execute_process(
...))` for installing the python bindings. apparently, this is not
expected. because the new shell's $PATH is very likely different
from the one changed by the building script. to address this, we
should always specify the `$LDSHARED` env var explicitly.

also, pass env vars using `ENV{}` instead of the `env` command to
workaround the issue of https://cmake.org/pipermail/cmake/2015-December/062216.html,
because it's not straightforward to set environment variables with
spaces in the them using cmake. and because one cannot use add_custom_target()
in the script mode of cmake. this leave me only limited options to
fix this issue.

Signed-off-by: Kefu Chai <kchai@redhat.com>