[cmake] Enable zlib support on windows

[labath]

Summary

zlib support was hard-wired to off for (non-cygwin) windows targets.
This disables some features, such as reading debug info from compressed
dwarf sections.

This has been this way since zlib support was added in 2013 (r180083),
but there is no obvious reason for that. Zlib is perfectly capable of
being compiled for windows (it even has a cmake file that works out of
the box).

This enables one to turn on zlib support on windows, if one has zlib
avaliable.

[beanz]

LGTM!

[rnk]

lgtm

[Hahnfeld]

I think this commit broke some buildbots:
http://lab.llvm.org:8011/builders/clang-x86_64-debian-fast/builds/7617
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-debian-fast/builds/6962 (this one was already red at that time, so you probably didn't get a notification)

Edit: Also reported on mailing list: http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20171127/506845.html

cmake/config-ix.cmake

if(HAVE_LIBPTHREAD)
   # have it reported as '-l<lib>' instead of '-pthread'.
   # TODO: switch to -pthread once the rest of the build system can deal with it.
   set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
   set(THREADS_HAVE_PTHREAD_ARG Off)
   find_package(Threads REQUIRED)
   set(LLVM_PTHREAD_LIB ${CMAKE_THREAD_LIBS_INIT})
 endif()
 
-# Don't look for these libraries on Windows. Also don't look for them if we're
-# using MSan, since uninstrumented third party code may call MSan interceptors
-# like strlen, leading to false positives.
-if( NOT PURE_WINDOWS AND NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
-  if (LLVM_ENABLE_ZLIB)
-    check_library_exists(z compress2 "" HAVE_LIBZ)
+# Don't look for these libraries if we're using MSan, since uninstrumented third
+# party code may call MSan interceptors like strlen, leading to false positives.
+if(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
+  find_library(ZLIB_LIBRARY_PATH NAMES z zlib)
+  if (LLVM_ENABLE_ZLIB AND ZLIB_LIBRARY_PATH)
+    check_library_exists(${ZLIB_LIBRARY_PATH} compress2 "" HAVE_LIBZ)
   else()
     set(HAVE_LIBZ 0)
   endif()
+
+  # Don't look for these libraries on Windows.
+  if (NOT PURE_WINDOWS)
     # Skip libedit if using ASan as it contains memory leaks.
     if (LLVM_ENABLE_LIBEDIT AND HAVE_HISTEDIT_H AND NOT LLVM_USE_SANITIZER MATCHES ".*Address.*")
       check_library_exists(edit el_init "" HAVE_LIBEDIT)
     else()
       set(HAVE_LIBEDIT 0)
     endif()
     if(LLVM_ENABLE_TERMINFO)
       set(HAVE_TERMINFO 0)
       foreach(library tinfo terminfo curses ncurses ncursesw)
         string(TOUPPER ${library} library_suffix)
         check_library_exists(${library} setupterm "" HAVE_TERMINFO_${library_suffix})
         if(HAVE_TERMINFO_${library_suffix})
           set(HAVE_TERMINFO 1)
           set(TERMINFO_LIBS "${library}")
           break()
         endif()
       endforeach()
     else()
       set(HAVE_TERMINFO 0)
     endif()
 
     find_library(ICONV_LIBRARY_PATH NAMES iconv libiconv libiconv-2 c)
     set(LLVM_LIBXML2_ENABLED 0)
     set(LIBXML2_FOUND 0)
     if((LLVM_ENABLE_LIBXML2) AND ((CMAKE_SYSTEM_NAME MATCHES "Linux") AND (ICONV_LIBRARY_PATH) OR APPLE))
       find_package(LibXml2)
       if (LIBXML2_FOUND)
         set(LLVM_LIBXML2_ENABLED 1)
         include_directories(${LIBXML2_INCLUDE_DIR})
         set(LIBXML2_LIBS "xml2")
       endif()
     endif()
   endif()
+endif()
 
 if (LLVM_ENABLE_LIBXML2 STREQUAL "FORCE_ON" AND NOT LLVM_LIBXML2_ENABLED)
   message(FATAL_ERROR "Failed to congifure libxml2")
 endif()
 
 check_library_exists(xar xar_open "" HAVE_LIBXAR)
 if(HAVE_LIBXAR)
   set(XAR_LIB xar)

lib/Support/CMakeLists.txt

 set(system_libs)
+if ( LLVM_ENABLE_ZLIB AND HAVE_LIBZ )
+  set(system_libs ${system_libs} ${ZLIB_LIBRARY_PATH})
+endif()
 if( MSVC OR MINGW )
   # libuuid required for FOLDERID_Profile usage in lib/Support/Windows/Path.inc.
   set(system_libs ${system_libs} psapi shell32 ole32 uuid)
 elseif( CMAKE_HOST_UNIX )
   if( HAVE_LIBRT )
     set(system_libs ${system_libs} rt)
   endif()
   if( HAVE_LIBDL )
     set(system_libs ${system_libs} ${CMAKE_DL_LIBS})
   endif()
   if( HAVE_BACKTRACE )
     set(system_libs ${system_libs} ${Backtrace_LIBRARIES})
   endif()
   if(LLVM_ENABLE_TERMINFO)
     if(HAVE_TERMINFO)
       set(system_libs ${system_libs} ${TERMINFO_LIBS})
     endif()
   endif()
   if( LLVM_ENABLE_THREADS AND HAVE_LIBATOMIC )
     set(system_libs ${system_libs} atomic)
   endif()
   set(system_libs ${system_libs} ${LLVM_PTHREAD_LIB})
-  if ( LLVM_ENABLE_ZLIB AND HAVE_LIBZ )
-    set(system_libs ${system_libs} z)
-  endif()
   if( UNIX AND NOT (BEOS OR HAIKU) )
     set(system_libs ${system_libs} m)
   endif()
 endif( MSVC OR MINGW )
 
 add_llvm_library(LLVMSupport
   AMDGPUMetadata.cpp
   APFloat.cpp