Enable lite-xl to be started from a symlink to the deployed binary (#249)

Use resolved executablePath instead of resourcePath to allow lanching
the lite-xl binary via a symlink, like typically done by Homebrew:

/usr/local/bin/lite-xl -> /Applications/lite-xl.app/Contents/MacOS/lite-xl

The resourcePath returns /usr/local in this case instead of
/Applications/lite-xl.app/Contents/Resources, which makes later
access to the resource files fail. Resolving the symlink to the
executable and then the relative path to the expected directory
Resources is a workaround for starting the application from both
the launcher directly and the command line via the symlink.

src/main.c

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <stdlib.h>
 #include <SDL.h>
 #include "api/api.h"
 #include "rencache.h"
@@ -62,8 +63,13 @@ static void get_exe_filename(char *buf, int sz) {
   int len = readlink(path, buf, sz - 1);
   buf[len] = '\0';
 #elif __APPLE__
+  /* use realpath to resolve a symlink if the process was launched from one.
+  ** This happens when Homebrew installs a cack and creates a symlink in
+  ** /usr/loca/bin for launching the executable from the command line. */
   unsigned size = sz;
-  _NSGetExecutablePath(buf, &size);
+  char exepath[size];
+  _NSGetExecutablePath(exepath, &size);
+  realpath(exepath, buf);
 #else
   strcpy(buf, "./lite");
 #endif