[CMAKE] Add ability to disable bindings

[mattico]

Summary

https://bugs.llvm.org/show_bug.cgi?id=36137

Building the Go and OCaml bindings occasionally causes build errors for downstream users. When these bindings are not used it is preferable to disable building them, rather than work around the build errors. The autoconf build system provided --disable-bindings, but there is no such option in the CMake system.

This commit adds this option, as LLVM_BUILD_BINDINGS:BOOL. Do let me know if some other configuration would be preferable. I made another version of this patch using an option like LLVM_BINDINGS_TO_BUILD:STRING, which was a much larger change.

I am not sure who to add as a reviewer, please suggest someone if you can.

[mattico]

Of course, I only notice the related revision after posting this one :)

See https://reviews.llvm.org/D42026

CMakeLists.txt

 option (LLVM_VERSION_PRINTER_SHOW_HOST_TARGET_INFO
   "Show target and host info when tools are invoked with --version." ON)
 
+option (LLVM_BUILD_BINDINGS
+  "Build bindings for Go and OCaml to LLVM." ON)  
+
 # You can configure which libraries from LLVM you want to include in the
 # shared library by setting LLVM_DYLIB_COMPONENTS to a semi-colon delimited
 # list of LLVM components. All component names handled by llvm-config are valid.

cmake/config-ix.cmake

   message(STATUS "Doxygen disabled.")
 endif()
 
-set(LLVM_BINDINGS "")
-if(WIN32)
-  message(STATUS "Go bindings disabled.")
-else()
-  find_program(GO_EXECUTABLE NAMES go DOC "go executable")
-  if(GO_EXECUTABLE STREQUAL "GO_EXECUTABLE-NOTFOUND")
-    message(STATUS "Go bindings disabled.")
-  else()
-    execute_process(COMMAND ${GO_EXECUTABLE} run ${PROJECT_SOURCE_DIR}/bindings/go/conftest.go
-                    RESULT_VARIABLE GO_CONFTEST)
-    if(GO_CONFTEST STREQUAL "0")
-      set(LLVM_BINDINGS "${LLVM_BINDINGS} go")
-      message(STATUS "Go bindings enabled.")
-    else()
-      message(STATUS "Go bindings disabled, need at least Go 1.2.")
-    endif()
-  endif()
-endif()
-
 find_program(GOLD_EXECUTABLE NAMES ${LLVM_DEFAULT_TARGET_TRIPLE}-ld.gold ld.gold ${LLVM_DEFAULT_TARGET_TRIPLE}-ld ld DOC "The gold linker")
 set(LLVM_BINUTILS_INCDIR "" CACHE PATH
   "PATH to binutils/include containing plugin-api.h for gold plugin.")

   endif()
 endif()
 
-# Keep the version requirements in sync with bindings/ocaml/README.txt.
-include(FindOCaml)
-include(AddOCaml)
-if(WIN32)
-  message(STATUS "OCaml bindings disabled.")
-else()
-  find_package(OCaml)
-  if( NOT OCAML_FOUND )
+if (LLVM_BUILD_BINDINGS)
+  set(LLVM_BINDINGS "")
+  if(WIN32)
+    message(STATUS "Go bindings disabled.")
+  else()
+    find_program(GO_EXECUTABLE NAMES go DOC "go executable")
+    if(GO_EXECUTABLE STREQUAL "GO_EXECUTABLE-NOTFOUND")
+      message(STATUS "Go bindings disabled.")
+    else()
+      execute_process(COMMAND ${GO_EXECUTABLE} run ${PROJECT_SOURCE_DIR}/bindings/go/conftest.go
+                      RESULT_VARIABLE GO_CONFTEST)
+      if(GO_CONFTEST STREQUAL "0")
+        set(LLVM_BINDINGS "${LLVM_BINDINGS} go")
+        message(STATUS "Go bindings enabled.")
+      else()
+        message(STATUS "Go bindings disabled, need at least Go 1.2.")
+      endif()
+    endif()
+  endif()
+
+  # Keep the version requirements in sync with bindings/ocaml/README.txt.
+  include(FindOCaml)
+  include(AddOCaml)
+  if(WIN32)
     message(STATUS "OCaml bindings disabled.")
   else()
-    if( OCAML_VERSION VERSION_LESS "4.00.0" )
-      message(STATUS "OCaml bindings disabled, need OCaml >=4.00.0.")
+    find_package(OCaml)
+    if( NOT OCAML_FOUND )
+      message(STATUS "OCaml bindings disabled.")
     else()
-      find_ocamlfind_package(ctypes VERSION 0.4 OPTIONAL)
-      if( HAVE_OCAML_CTYPES )
-        message(STATUS "OCaml bindings enabled.")
-        find_ocamlfind_package(oUnit VERSION 2 OPTIONAL)
-        set(LLVM_BINDINGS "${LLVM_BINDINGS} ocaml")
-
-        set(LLVM_OCAML_INSTALL_PATH "${OCAML_STDLIB_PATH}" CACHE STRING
-            "Install directory for LLVM OCaml packages")
+      if( OCAML_VERSION VERSION_LESS "4.00.0" )
+        message(STATUS "OCaml bindings disabled, need OCaml >=4.00.0.")
       else()
-        message(STATUS "OCaml bindings disabled, need ctypes >=0.4.")
+        find_ocamlfind_package(ctypes VERSION 0.4 OPTIONAL)
+        if( HAVE_OCAML_CTYPES )
+          message(STATUS "OCaml bindings enabled.")
+          find_ocamlfind_package(oUnit VERSION 2 OPTIONAL)
+          set(LLVM_BINDINGS "${LLVM_BINDINGS} ocaml")
+
+          set(LLVM_OCAML_INSTALL_PATH "${OCAML_STDLIB_PATH}" CACHE STRING
+              "Install directory for LLVM OCaml packages")
+        else()
+          message(STATUS "OCaml bindings disabled, need ctypes >=0.4.")
+        endif()
       endif()
     endif()
   endif()
-endif()
 
   string(REPLACE " " ";" LLVM_BINDINGS_LIST "${LLVM_BINDINGS}")
+else()
+  message(STATUS "Bindings disabled.")
+endif()
 
 function(find_python_module module)
   string(REPLACE "." "_" module_name ${module})

docs/CMake.rst

   either be absolute or relative to the CMAKE_INSTALL_PREFIX. Defaults to
   `share/doc/llvm/ocaml-html`.
 
+**LLVM_BUILD_BINDINGS**:BOOL
+  If enabled, the targets for building the Go and OCaml bindings are added.
+  Either binding may not build if dependencies are not found. Defaults to ON.
+
 **LLVM_CREATE_XCODE_TOOLCHAIN**:BOOL
   OS X Only: If enabled CMake will generate a target named
   'install-xcode-toolchain'. This target will create a directory at