FindZLIB: fix CMAKE_FIND_LIBRARY_PREFIXES being unset when it was empty

CMAKE_FIND_LIBRARY_PREFIXES and CMAKE_FIND_LIBRARY_SUFFIXES have
different behavior when undefined and when defined but empty:
Empty means to use an empty prefix/suffix while undefined means to
use a hardcoded default for the platform we are running on.

Unfortunately, set(a ${b}) will undefine a when b is empty,
meaning that when targeting a platform where either of these variables
is empty (e.g. Windows where CMAKE_FIND_LIBRARY_PREFIXES is empty)
the unpatched FindZLIB code ends up unsetting that variable, causing
all subsequent find_library calls to use the hardcoded default
for the runtime platform (e.g. "lib" for CMAKE_FIND_LIBRARY_PREFIXES
on Linux).

On the other hand, set(a "${b}") will always define a to be empty but
defined so we have to do this dance to fully preserve the state of these
variables.

0 files changed, 0 insertions, 0 deletions