all: fix build with musl libc

- Should not assume whether O_xxx macros are defined by checking
  feature test macros.
- Workaround conflicting getdents() declaration.

src/defs.h

@@ -97,9 +97,6 @@
 
 #if defined(__FreeBSD__) || (defined(__linux__) && defined(_GNU_SOURCE))
 #  define HAVE_PIPE2  1
-#else
-#  define O_DIRECT  0
-#  define O_PATH    0  // FreeBSD supports O_PATH since 13.1
 #endif
 
 #ifndef __FreeBSD__

src/fs_ops.c

@@ -794,9 +794,11 @@ bm_open (
         }
     }
 
+#ifdef O_DIRECT
     if (flags & O_DIRECT) {
         fi->direct_io = 1;
     }
+#endif
 #ifndef __FreeBSD__
     // Cannot reliably keep cache on FreeBSD, we're unable to explicitly
     // flush it.  See comments for inval_inode().

src/fsck_online.c

@@ -68,7 +68,7 @@ struct fsck_dir {
 
 // Forward declaration start
 #ifdef __linux__
-static ssize_t getdents (int, void *, size_t);
+static ssize_t getdents_ (int, void *, size_t);
 #endif
 
 static int  next_subdir   (struct fsck_ctx *, struct fsck_dir *,
@@ -81,8 +81,11 @@ static void print_version (void);
 
 #ifdef __linux__
 
+// Some libc (e.g., musl) may declare a getdents() function
+// in dirent.h with conflicting types.
+#define getdents getdents_
 static ssize_t
-getdents (
+getdents_ (
     int     dirfd,
     void   *buf,
     size_t  bufsize

src/watcher.c

@@ -158,8 +158,11 @@ impl_rearm (
     }
 
 #elif defined(WATCHER_IMPL_KQUEUE)
-    int wfd = openat(w->dirfd, w->name,
-            O_RDONLY | O_CLOEXEC | O_PATH | O_RESOLVE_BENEATH);
+    int open_flags = O_RDONLY | O_CLOEXEC | O_RESOLVE_BENEATH;
+#ifdef O_PATH
+    open_flags |= O_PATH;
+#endif
+    int wfd = openat(w->dirfd, w->name, open_flags);
     if (wfd < 0) {
         log_printf("openat(): %s", xstrerror(errno));
         return -1;