Another attempt at dealing with cython 3.0.0.

kernel/headers/SnapPea.h

@@ -497,10 +497,11 @@ typedef struct NormalSurfaceList            NormalSurfaceList;
 #define CONST
 /* #define CONST const */
 
-
+#ifdef FORCE_C_LINKAGE
 #ifdef __cplusplus
 extern "C" {
 #endif
+#endif
 
 /************************************************************************/
 
@@ -2731,9 +2732,11 @@ extern Real volume(Triangulation *manifold, int *precision);
 
 #include "end_namespace.h"
 
+#ifdef FORCE_C_LINKAGE
 #ifdef __cplusplus
 }
 #endif
+#endif
 
 #endif
 /* Local Variables:                      */

kernel/headers/kernel.h

@@ -11,9 +11,11 @@
 
 #include "SnapPea.h"
 
+#ifdef FORCE_C_LINKAGE
 #ifdef __cplusplus
 extern "C" {
 #endif
+#endif
 
 #include <string.h>
 #include <math.h>
@@ -30,9 +32,11 @@ extern "C" {
 #include "kernel_prototypes.h"
 #include "tables.h"
 
+#ifdef FORCE_C_LINKAGE
 #ifdef __cplusplus
 }
 #endif
+#endif
 
 #endif
 /* Local Variables:                      */

setup.py

@@ -396,6 +396,12 @@ def add(self, source_file, dependency_mod_time=0.0):
     elif platform.machine() == 'x86_64':
         hp_extra_compile_args = ['-mfpmath=sse', '-msse2', '-mieee-fp']
 
+if have_cython:
+    if [int(x) for x in cython_version.split('.')[:2]] < [3, 0]:
+        if sys.platform == 'win32':
+            hp_extra_compile_args.append('/DFORCE_C_LINKAGE')
+        else:
+            hp_extra_compile_args.append('-DFORCE_C_LINKAGE')
 
 # SnapPyHP depends implicitly on the source for the main kernel, so we
 # we delete certain object files to force distutils to rebuild them.