build: don't allow Xcode as generator

Xcode does not allow having multiple targets depend on a custom command.
This limitation severely hinders its usability and complying with it
would likely require extensive refactoring and boilerplate. It makes
more sense to simply refer users to use "Ninja" or "Unix Makefiles"
instead.

BUILD.md

@@ -205,18 +205,6 @@ echo '#include "./src/nvim/buffer.h"' | \
 - `grep -v /usr/` is used to filter out system header files.
 - `-save-temps` can be added as well to see expanded macros or commented assembly.
 
-## Xcode and MSVC project files
-
-CMake has a `-G` option for exporting to multiple [project file formats](http://www.cmake.org/cmake/help/v2.8.8/cmake.html#section_Generators), such as Xcode and Visual Studio. 
-
-For example, to use Xcode's static analysis GUI ([#167](https://github.com/neovim/neovim/issues/167#issuecomment-36136018)), you need to generate an Xcode project file from the Neovim Makefile (where `neovim/` is the top-level Neovim source code directory containing the main `Makefile`):
-
-```
-cmake -G Xcode neovim
-```
-
-The resulting project file can then be opened in Xcode.
-
 ## Custom Makefile
 
 You can customize the build process locally by creating a `local.mk`, which is referenced at the top of the main `Makefile`. It's listed in `.gitignore`, so it can be used across branches. **A new target in `local.mk` overrides the default make-target.**
@@ -265,7 +253,7 @@ cmake --build build
    cmake -B build -G Ninja -D CMAKE_BUILD_TYPE=RelWithDebInfo
    cmake --build build
    ```
-3. Run `make`, `ninja`, or whatever build tool you [told CMake to generate](#xcode-and-msvc-project-files).
+3. Run `make`, `ninja`, or whatever build tool you told CMake to generate.
     - Using `ninja` is strongly recommended.
 
 #### Debian 10 (Buster) example:

CMakeLists.txt

@@ -17,6 +17,10 @@ if(POLICY CMP0135)
   cmake_policy(SET CMP0135 NEW)
 endif()
 
+if(XCODE)
+  message(FATAL_ERROR [[Xcode generator is not supported. Use "Ninja" or "Unix Makefiles" instead.]])
+endif()
+
 # Point CMake at any custom modules we may ship
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake")