CMake: Turn LLVM_ENABLE_ZLIB into a tri-state option

[akhuang]

Summary

Add FORCE_ON option to LLVM_ENABLE_ZLIB, which causes a configuration
error if zlib is not found.
Similar to https://reviews.llvm.org/D40050.

[rnk]

lgtm

[grimar]

llvm/CMakeLists.txt

351

I wonder if REQUIRE sounds better.
(For me it was not clear what FORCE_ON means from its name).

But, it is not consistent with the existent FORCE_ON used for libxml2.

llvm/CMakeLists.txt

 set(LLVM_ENABLE_LIBXML2 "ON" CACHE STRING "Use libxml2 if available. Can be ON, OFF, or FORCE_ON")
 
 option(LLVM_ENABLE_LIBEDIT "Use libedit if available." ON)
 
 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.")
 
 find_package(Z3 4.7.1)
 
 if (LLVM_Z3_INSTALL_DIR)
   if (NOT Z3_FOUND)
     message(FATAL_ERROR "Z3 >= 4.7.1 has not been found in LLVM_Z3_INSTALL_DIR: ${LLVM_Z3_INSTALL_DIR}.")

llvm/cmake/config-ix.cmake

if (NOT PURE_WINDOWS)
     endif()
   endif()
 endif()
 
 if (LLVM_ENABLE_LIBXML2 STREQUAL "FORCE_ON" AND NOT LLVM_LIBXML2_ENABLED)
   message(FATAL_ERROR "Failed to congifure libxml2")
 endif()
 
+if (LLVM_ENABLE_ZLIB STREQUAL "FORCE_ON" AND NOT HAVE_LIBZ)
+  message(FATAL_ERROR "Failed to configure zlib")
+endif()
+
 check_library_exists(xar xar_open "" HAVE_LIBXAR)
 if(HAVE_LIBXAR)
   set(XAR_LIB xar)
 endif()
 
 # function checks
 check_symbol_exists(arc4random "stdlib.h" HAVE_DECL_ARC4RANDOM)
 find_package(Backtrace)