Extracted fix from postfix-3.8-20221006 snapshot:
    https://github.com/vdukhovni/postfix/commit/b65530350fa4a7eee40946160fd80c3e1e0b63e5
--- a/makedefs
+++ b/makedefs
@@ -627,7 +627,8 @@ EOF
 		: ${SHLIB_ENV="LD_LIBRARY_PATH=`pwd`/lib"}
 		: ${PLUGIN_LD="${CC-gcc} -shared"}
 		;;
- Linux.[345].*)	SYSTYPE=LINUX$RELEASE_MAJOR
+    Linux.[3456].*)
+		SYSTYPE=LINUX$RELEASE_MAJOR
 		case "$CCARGS" in
 		 *-DNO_DB*) ;;
 		 *-DHAS_DB*) ;;
--- a/src/util/sys_defs.h
+++ b/src/util/sys_defs.h
@@ -751,7 +751,8 @@ extern int initgroups(const char *, int);
  /*
   * LINUX.
   */
-#if defined(LINUX2) || defined(LINUX3) || defined(LINUX4) || defined(LINUX5)
+#if defined(LINUX2) || defined(LINUX3) || defined(LINUX4) || defined(LINUX5) \
+	|| defined(LINUX6)
 #define SUPPORTED
 #define UINT32_TYPE	unsigned int
 #define UINT16_TYPE	unsigned short