Use a separate switch for system zstd with cffi

Given that the CFFI code is much more fragile to library changes
than the C extension code, use a separate `--system-zstd-cffi` switch
to enable it.  This should resolve the test failure on macOS.

Signed-off-by: Michał Górny <mgorny@gentoo.org>

Author: mgorny

.github/workflows/external-zstd.yml

@@ -29,7 +29,7 @@ jobs:
 
       - name: Build
         run: |
-          python -m pip install --config-settings=--global-option=--system-zstd .
+          python -m pip install --config-settings=--global-option=--system-zstd --config-settings=--global-option=--system-zstd-cffi .
 
   macOS:
     runs-on: 'macos-13'

setup.py

@@ -73,6 +73,7 @@
 
 SUPPORT_LEGACY = False
 SYSTEM_ZSTD = False
+SYSTEM_ZSTD_CFFI = False
 WARNINGS_AS_ERRORS = False
 C_BACKEND = True
 CFFI_BACKEND = True
@@ -103,6 +104,10 @@
     SYSTEM_ZSTD = True
     sys.argv.remove("--system-zstd")
 
+if "--system-zstd-cffi" in sys.argv:
+    SYSTEM_ZSTD_CFFI = True
+    sys.argv.remove("--system-zstd-cffi")
+
 if "--warnings-as-errors" in sys.argv:
     WARNINGS_AS_ERRORS = True
     sys.argv.remove("--warning-as-errors")
@@ -138,7 +143,7 @@
 if CFFI_BACKEND and cffi:
     import make_cffi
 
-    extensions.append(make_cffi.get_ffi(system_zstd=SYSTEM_ZSTD).distutils_extension())
+    extensions.append(make_cffi.get_ffi(system_zstd=SYSTEM_ZSTD_CFFI).distutils_extension())
 
 version = None