Ben Keller authored May 10, 2016 and Brad King committed Sep 06, 2016

When exporting from a project (with install(EXPORT ...)), the
`<PROJECT>Targets.cmake` file contains logic for computing the
`_IMPORT_PREFIX` from its own location.  This `_IMPORT_PREFIX` is then
used in the `<PROJECT>Targets-<config>.cmake` file to generate the
`IMPORTED_LOCATION_<CONFIG>`.  The generation unconditionally appends a
"/" to `_IMPORT_PREFIX` before appending the rest of the path.  If
`_IMPORT_PREFIX` is "/", then the `IMPORTED_LOCATION_<CONFIG>`
properties all start with exactly two leading slashes ("//").

Exactly two leading slashes is a special case in POSIX file paths, such
that its interpretation is left up to the implementation.  This means
that changing the path prefix from "/" to "//" should not be allowed.

Since references to `_IMPORT_PREFIX` are always followed by a "/",
simply check the value to replace "/" with "".