musl: basename: use portable implementation for basename API

musl has removed the non-prototype declaration of basename from string.h which
now results in build errors with newer clang compilers.

Implement GNU basename behavior using strchr which is portable across libcs.

Fixes:
| ../../git/tools/mountcomposefs.c:43:20:
| error: call to undeclared function 'basename'; ISO C99 and later do not
| support implicit function declarations [-Wimplicit-function-declaration]
|    43 |         const char *bin = basename(argv0);
|       |                           ^
| ../../git/tools/mountcomposefs.c:43:14:
| error: incompatible integer to pointer conversion initializing 'const char *'
| with an expression of type 'int' [-Wint-conversion]
|    43 |         const char *bin = basename(argv0);
|       |                     ^     ~~~~~~~~~~~~~~~

For reference:
https://git.musl-libc.org/cgit/musl/commit/?id=725e17ed6dff4d0cd22487bb64470881e86a92e7

Closes: #272

Signed-off-by: Fathi Boudra <[email protected]>

libcomposefs/lcfs-utils.h

@@ -161,4 +161,10 @@ static inline void *steal_pointer(void *pp)
 /* type safety */
 #define steal_pointer(pp) (0 ? (*(pp)) : (steal_pointer)(pp))
 
+static inline const char *gnu_basename(const char *filename)
+{
+	const char *p = strrchr(filename, '/');
+	return p ? p+1 : filename;
+}
+
 #endif

tools/mkcomposefs.c

@@ -945,7 +945,7 @@ static void digest_to_string(const uint8_t *csum, char *buf)
 
 static void usage(const char *argv0)
 {
-	const char *bin = basename(argv0);
+	const char *bin = gnu_basename(argv0);
 	fprintf(stderr,
 		"Usage: %s [OPTIONS] SOURCE IMAGE\n"
 		"Options:\n"

tools/mountcomposefs.c

@@ -37,10 +37,11 @@
 #include <linux/fsverity.h>
 
 #include "libcomposefs/lcfs-mount.h"
+#include "libcomposefs/lcfs-utils.h"
 
 static void usage(const char *argv0)
 {
-	const char *bin = basename(argv0);
+	const char *bin = gnu_basename(argv0);
 	fprintf(stderr,
 		"usage: %s [-t type] [-o opt[,opts..]] IMAGE MOUNTPOINT\n"
 		"Example:\n"