[libc++] Fix a few incorrect CMake configuration options

This patch fixes a few CMake options that were set using incorrect
mechanisms.

CMake's man page for the -D <var>=<value> option states: If a command in
the project sets the type to PATH or FILEPATH, then the <value> will be
converted to an absolute path. That's not what we want for most of the
paths we have as configuration options. Otherwise, using -D to set the
configuration option results in an absolute path being used, which
breaks things.

option() denotes a boolean variable, but what was desired was a
string/list variable. Fix this to prevent cmake from changing any
non-empty user provided values to 'ON'.

Differential Revision: https://reviews.llvm.org/D157926