Use portable implementation for basename API

musl has removed the non-prototype declaration of basename from
string.h [1] which now results in build errors with clang-17+ compiler

Implement GNU basename behavior using strchr which is portable across libcs

Fixes
../git/tools/kmod.c:71:19: error: call to undeclared function 'basename'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
71 | "Commands:\n", basename(argv[0]));
| ^

[1] https://git.musl-libc.org/cgit/musl/commit/?id=725e17ed6dff4d0cd22487bb64470881e86a92e7

Suggested-by: Rich Felker

Signed-off-by: Khem Raj <raj.khem@gmail.com>
[ Implement a basename() function in missing.h and ensure we always use
  the right include rather than having a separate gnu_basename() ]
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>

configure.ac

@@ -53,7 +53,9 @@ CC_CHECK_FUNC_BUILTIN([__builtin_uaddll_overflow], [ ], [ ])
 AC_CHECK_MEMBERS([struct stat.st_mtim], [], [], [#include <sys/stat.h>])
 
 # musl 1.0 and bionic 4.4 don't have strndupa
-AC_CHECK_DECLS_ONCE([strndupa])
+# basename may be only available in libgen.h with the POSIX behavior,
+# not desired here
+AC_CHECK_DECLS_ONCE([[strndupa], [basename]], [], [], [[#include <string.h>]])
 
 # RHEL 5 and older do not have be32toh
 AC_CHECK_DECLS_ONCE([be32toh])

shared/missing.h

@@ -38,6 +38,7 @@ static inline int finit_module(int fd, const char *uargs, int flags)
 #endif
 
 #if !HAVE_DECL_STRNDUPA
+#include <string.h>
 #define strndupa(s, n)							\
 	({								\
 		const char *__old = (s);				\
@@ -48,6 +49,15 @@ static inline int finit_module(int fd, const char *uargs, int flags)
 	 })
 #endif
 
+#if !HAVE_DECL_BASENAME
+#include <string.h>
+static inline const char *basename(const char *s)
+{
+	const char *p = strrchr(s, '/');
+	return p ? p + 1 : s;
+}
+#endif
+
 #if !HAVE_DECL_BE32TOH
 #include <endian.h>
 #include <byteswap.h>

shared/util.c

@@ -172,7 +172,7 @@ char *modname_normalize(const char *modname, char buf[static PATH_MAX], size_t *
 
 char *path_to_modname(const char *path, char buf[static PATH_MAX], size_t *len)
 {
-	char *modname;
+	const char *modname;
 
 	modname = basename(path);
 	if (modname == NULL || modname[0] == '\0')

shared/util.h

@@ -5,6 +5,7 @@
 #include <stdbool.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <time.h>

tools/kmod.c

@@ -24,6 +24,7 @@
 #include <string.h>
 
 #include <shared/util.h>
+#include <shared/missing.h>
 
 #include <libkmod/libkmod.h>