Improve compatibility of calls to dirname() on constant strings.

As the xinstall(8) utility had to be patched up to work with the POSIXly
correct basename()/dirname() prototypes, we make it pretty hard to build
previous versions of FreeBSD on HEAD. xinstall(8) is part of the
bootstrap tools.

Add some logic to <libgen.h> to automatically detect bad calls to
dirname() based on the type of the argument. If the argument is of type
'const char *', we simply fall back to calling into dirname@FBSD_1.0
directly.

I'll also give basename() similar treatment when importing the
thread-safe version of that function.

Tested by:	bdrewery, madpilot (thanks!)

include/libgen.h

@@ -39,4 +39,23 @@ char	*basename_r(const char *, char *);
 char	*dirname(char *);
 __END_DECLS
 
+/*
+ * In FreeBSD 12, the prototype of dirname() was modified to comply to
+ * POSIX. This function may now modify its input. Unfortunately, our
+ * copy of xinstall(8) shipped with previous versions of FreeBSD is
+ * built using the host headers and libc during the bootstrapping phase
+ * and depends on the old behavior.
+ *
+ * Apply a workaround where we explicitly link against dirname@FBSD_1.0
+ * in case this function is called on constant strings, instead of
+ * making the build fail.
+ */
+#if defined(__generic) && !defined(__cplusplus)
+__BEGIN_DECLS
+char	*__old_dirname(const char *);
+__END_DECLS
+__sym_compat(dirname, __old_dirname, FBSD_1.0);
+#define	dirname(x)	__generic(x, const char *, __old_dirname, dirname)(x)
+#endif
+
 #endif /* !_LIBGEN_H_ */

lib/libc/gen/dirname.c

@@ -31,7 +31,7 @@ __FBSDID("$FreeBSD$");
 #include <string.h>
 
 char *
-dirname(char *path)
+(dirname)(char *path)
 {
 	const char *in, *prev, *begin, *end;
 	char *out;