On Linux, uses external libffi for python >= 3.7

Since python/cpython@f40d4ddff (Closes python/cpython#27979: Remove
bundled copy of libffi), external version of libffi is required
on linux.

Author: Dan Dees

cmake/extensions/CMakeLists.txt

@@ -330,47 +330,60 @@ else()
             DEFINITIONS MACOSX
         )
     else()
-        set(_libffi_sources)
-        set(_libffi_include_dirs)
         if(PY_VERSION VERSION_LESS "3.7")
-            if("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86_64")
-                set(_libffi_system_extra_src
-                    _ctypes/libffi/src/${_libffi_system_dir}/ffi64.c
-                    _ctypes/libffi/src/${_libffi_system_dir}/unix64.S
+            set(_libffi_sources)
+            set(_libffi_include_dirs)
+            if(PY_VERSION VERSION_LESS "3.7")
+                if("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86_64")
+                    set(_libffi_system_extra_src
+                        _ctypes/libffi/src/${_libffi_system_dir}/ffi64.c
+                        _ctypes/libffi/src/${_libffi_system_dir}/unix64.S
+                        )
+                elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "i686")
+                    set(_libffi_system_extra_src
+                        _ctypes/libffi/src/${_libffi_system_dir}/win32.S
+                        )
+                endif()
+                # To facilitate an eventual contribution of the configuration
+                # of fficonfig.h to the upstream project, corresponding tests
+                # in ConfigureChecks.cmake are labeled using this convention:
+                # * "libffi specific"
+                # * "libffi and cpython"
+                set(LIBFFI_VERSION "3.1")
+                configure_file(
+                    ${PROJECT_SOURCE_DIR}/cmake/fficonfig.h.in
+                    ${INCLUDE_BUILD_DIR}/fficonfig.h
                     )
-            elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "i686")
-                set(_libffi_system_extra_src
-                    _ctypes/libffi/src/${_libffi_system_dir}/win32.S
+                set(_libffi_sources
+                    _ctypes/libffi/src/closures.c
+                    _ctypes/libffi/src/prep_cif.c
+                    _ctypes/libffi/src/${_libffi_system_dir}/ffi.c
+                    _ctypes/libffi/src/${_libffi_system_dir}/sysv.S
+                    ${_libffi_system_extra_src}
+                    )
+                set(_libffi_include_dirs
+                    INCLUDEDIRS ${SRC_DIR}/Modules/_ctypes/libffi/src/${_libffi_system_dir}
+                            ${SRC_DIR}/Modules/_ctypes/libffi/include
+                            ${INCLUDE_BUILD_DIR}      # For fficonfig.h
+                            ${PROJECT_SOURCE_DIR}/cmake # For ffi.h
                     )
             endif()
-            # To facilitate an eventual contribution of the configuration
-            # of fficonfig.h to the upstream project, corresponding tests
-            # in ConfigureChecks.cmake are labeled using this convention:
-            # * "libffi specific"
-            # * "libffi and cpython"
-            set(LIBFFI_VERSION "3.1")
-            configure_file(
-                ${PROJECT_SOURCE_DIR}/cmake/fficonfig.h.in
-                ${INCLUDE_BUILD_DIR}/fficonfig.h
-                )
-            set(_libffi_sources
-                _ctypes/libffi/src/closures.c
-                _ctypes/libffi/src/prep_cif.c
-                _ctypes/libffi/src/${_libffi_system_dir}/ffi.c
-                _ctypes/libffi/src/${_libffi_system_dir}/sysv.S
-                ${_libffi_system_extra_src}
-                )
-            set(_libffi_include_dirs
-                INCLUDEDIRS ${SRC_DIR}/Modules/_ctypes/libffi/src/${_libffi_system_dir}
-                        ${SRC_DIR}/Modules/_ctypes/libffi/include
-                        ${INCLUDE_BUILD_DIR}      # For fficonfig.h
-                        ${PROJECT_SOURCE_DIR}/cmake # For ffi.h
-                )
+            add_python_extension(_ctypes
+                SOURCES ${ctypes_COMMON_SOURCES} ${_libffi_sources}
+                ${_libffi_include_dirs}
+            )
+
+        else()
+            # get libffi
+            find_path(FFI_INCLUDE_DIR ffi.h)
+            find_library(FFI_LIBRARY NAMES ffi libffi)
+            add_python_extension(_ctypes
+                REQUIRES FFI_INCLUDE_DIR FFI_LIBRARY
+                SOURCES ${ctypes_COMMON_SOURCES}
+                INCLUDEDIRS ${FFI_INCLUDE_DIR}
+                LIBRARIES ${FFI_LIBRARY}
+            )
         endif()
-        add_python_extension(_ctypes
-            SOURCES ${ctypes_COMMON_SOURCES} ${_libffi_sources}
-            ${_libffi_include_dirs}
-        )
     endif()
 endif()