mandoc: fix bootstrapping on Linux with Clang 16

It appears that Clang 16 errors by default for the mismatched function
pointer types that are triggered by the fts callback (since glibc has a
callback type without the second const qualifier).
Fortunately, there is already code to handle glibc inside mandoc, we
just have to edit the checked-in config.h.

Reviewed By:	imp, emaste
Differential Revision: https://reviews.freebsd.org/D42999

contrib/mandoc/config.h

@@ -13,7 +13,11 @@
 #define HAVE_ENDIAN 0
 #define HAVE_ERR 1
 #define HAVE_FTS 1
+#ifdef __GLIBC__
+#define HAVE_FTS_COMPARE_CONST 0
+#else
 #define HAVE_FTS_COMPARE_CONST 1
+#endif
 #define HAVE_GETLINE 1
 #define HAVE_GETSUBOPT 1
 #define HAVE_ISBLANK 1