docs: add info

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

README.md

@@ -15,6 +15,72 @@
 
 <!-- SPHINX-START -->
 
+This provides helpers for using Cython. Use:
+
+```cmake
+find_package(Cython MODULE REQUIRED VERSION 3.0)
+```
+
+If you find Python beforehand, the search will take this into account. You can
+specify a version range on CMake 3.19+. This will define a `Cython::Cython`
+target (along with a matching `CYTHON_EXECUTABLE` variable). It will also
+provide the following helper function:
+
+```cmake
+Cython_compile_pyx(<pyx_file>
+                  [LANGUAGE C | CXX]
+                  [CYTHON_ARGS <args> ...]
+                  [OUTPUT <OutputFile>]
+                  [OUTPUT_VARIABLE <OutputVariable>]
+                  )
+```
+
+This function takes a pyx file and makes a matching `.c` / `.cxx` file in the
+current binary directory (exact path can be specified with `OUTPUT`). The
+location of the produced file is placed in the variable specified by
+`OUTPUT_VARIABLE` if given. Extra arguments to the Cython executable can be
+given with `CYTHON_ARGS`, and if this is not set, it will take a default from a
+`CYTHON_ARGS` variable.
+
+If the `LANGUAGE` is not given, and both `C` and `CXX` are enabled globally,
+then the language will try to be deduced from a `# distutils: language=...`
+comment in the source file. It is an error if it cannot be deduced.
+
+This utility relies on the `DEPFILE` feature introduced for Ninja in CMake 3.7,
+and added for Make in CMake 3.20, and Visual Studio & Xcode in CMake 3.21.
+
+## Example
+
+```cmake
+find_package(
+  Python
+  COMPONENTS Interpreter Development.Module
+  REQUIRED)
+find_package(Cython MODULE REQUIRED)
+
+cython_compile_pyx(simple.pyx LANGUAGE C OUTPUT_VARIABLE simple_c)
+
+python_add_library(simple MODULE "${simple_c}" WITH_SOABI)
+```
+
+## scikit-build-core
+
+To use this package with scikit-build-core, you need to include it in your build
+requirements:
+
+```toml
+[build-system]
+requires = ["scikit-build-core", "cython", "cython-cmake"]
+build-backend = "scikit_build_core.build"
+```
+
+It is also recommended to require CMake 3.21:
+
+```toml
+[tool.scikit-build]
+cmake.version = ">=3.21"
+```
+
 <!-- prettier-ignore-start -->
 [actions-badge]:            https://github.com/scikit-build/cython-cmake/workflows/CI/badge.svg
 [actions-link]:             https://github.com/scikit-build/cython-cmake/actions

src/cython_cmake/cmake/UseCython.cmake

@@ -73,7 +73,7 @@
 #=============================================================================
 
 if(CMAKE_VERSION VERSION_LESS "3.7")
-  message(SEND_ERROR "CMake 3.7 required for DEPFILE")
+  message(FATAL_ERROR "CMake 3.7 required for DEPFILE")
 endif()
 
 
@@ -179,7 +179,6 @@ function(Cython_compile_pyx)
     COMMENT
     "Cythonizing source ${input_file_relative} to output ${generated_file_relative}"
   )
-
 endfunction()
 
 function(_cython_compute_language OUTPUT_VARIABLE FILENAME)