Teach find_library and find_package to search lib32 paths (#11260)

Add a ``FIND_LIBRARY_USE_LIB32_PATHS`` global property analogous to the
``FIND_LIBRARY_USE_LIB64_PATHS`` property.  This helps find commands on
multilib systems that use ``lib32`` directories and either do not have
``lib`` symlinks or point ``lib`` to ``lib64``.

Modules/Platform/Linux.cmake

@@ -50,8 +50,9 @@ set(CMAKE_LIBRARY_ARCHITECTURE_REGEX "[a-z0-9_]+(-[a-z0-9_]+)?-linux-gnu[a-z0-9_
 
 include(Platform/UnixPaths)
 
-# Debian has lib64 paths only for compatibility so they should not be
+# Debian has lib32 and lib64 paths only for compatibility so they should not be
 # searched.
 if(NOT CMAKE_CROSSCOMPILING AND EXISTS "/etc/debian_version")
+  set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB32_PATHS FALSE)
   set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB64_PATHS FALSE)
 endif()

Modules/Platform/OpenBSD.cmake

@@ -18,6 +18,7 @@ endif()
 set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_OPENBSD_VERSIONING 1)
 
 # OpenBSD has no multilib
+set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB32_PATHS FALSE)
 set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB64_PATHS FALSE)
 
 # OpenBSD policy requires that shared libraries be installed without

Modules/Platform/UnixPaths.cmake

@@ -93,5 +93,6 @@ list(APPEND CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES
   /usr/include
   )
 
-# Enable use of lib64 search path variants by default.
+# Enable use of lib32 and lib64 search path variants by default.
+set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB32_PATHS TRUE)
 set_property(GLOBAL PROPERTY FIND_LIBRARY_USE_LIB64_PATHS TRUE)