build: make LLVM_ENABLE_ZLIB a tri-bool for users

Treat the flag `LLVM_ENABLE_ZLIB` as a tri-bool, `FORCE_ON` being `ON`,
and `ON` being an auto-detect.  This is needed as many of the builders
enable the flag without having zlib available.

llvm/CMakeLists.txt

@@ -347,7 +347,7 @@ option(LLVM_ENABLE_LIBPFM "Use libpfm for performance counters if available." ON
 
 option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 
-option(LLVM_ENABLE_ZLIB "Use zlib for compression/decompression if available." ON)
+set(LLVM_ENABLE_ZLIB "ON" CACHE STRING "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON")
 
 set(LLVM_Z3_INSTALL_DIR "" CACHE STRING "Install directory of the Z3 solver.")
 

llvm/cmake/config-ix.cmake

@@ -117,10 +117,6 @@ endif()
 # 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.*")
-  if(LLVM_ENABLE_ZLIB)
-    find_package(ZLIB REQUIRED)
-  endif()
-
   # Don't look for these libraries on Windows.
   if (NOT PURE_WINDOWS)
     # Skip libedit if using ASan as it contains memory leaks.
@@ -506,7 +502,21 @@ else( LLVM_ENABLE_THREADS )
 endif()
 
 if(LLVM_ENABLE_ZLIB)
-  find_package(ZLIB REQUIRED)
+  if(LLVM_ENABLE_ZLIB STREQUAL FORCE_ON)
+    find_package(ZLIB REQUIRED)
+  else()
+    find_package(ZLIB)
+  endif()
+
+  if(ZLIB_FOUND)
+    set(LLVM_ENABLE_ZLIB "YES" CACHE STRING
+      "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON"
+      FORCE)
+  else()
+    set(LLVM_ENABLE_ZLIB "NO" CACHE STRING
+      "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON"
+      FORCE)
+  endif()
 endif()
 
 if (LLVM_ENABLE_DOXYGEN)

llvm/lib/Support/CMakeLists.txt

@@ -1,4 +1,7 @@
-set(system_libs ${ZLIB_LIBRARY})
+if(LLVM_ENABLE_ZLIB)
+  set(system_libs ${ZLIB_LIBRARY})
+endif()
+
 if( MSVC OR MINGW )
   # libuuid required for FOLDERID_Profile usage in lib/Support/Windows/Path.inc.
   # advapi32 required for CryptAcquireContextW in lib/Support/Windows/Path.inc.