Resolve a symlinked java command in path

Part of fixes for #34 and jruby/jruby#6637. This patch only adds
resolution to a `java` command found in PATH.

Author: headius

unixlauncher.cpp

@@ -57,6 +57,7 @@ int UnixLauncher::run(int argc, char* argv[], char* envp[]) {
         } else {
             java = findOnPath("java");
             if (!java.empty()) {
+                java = resolveSymlinks(java);
                 int home_index = java.find_last_of('/', java.find_last_of('/') - 1);
                 jdkhome = java.substr(0, home_index);
             }

utilsfuncs.cpp

@@ -146,6 +146,20 @@ string findOnPath(const char* name) {
     return "";
 }
 
+string resolveSymlinks(string path) {
+    struct stat st;
+    char tmp[PATH_MAX + 1];
+
+    lstat(path.c_str(), &st);
+    if (st.st_mode & S_IFLNK) {
+        realpath(path.c_str(), tmp);
+    }
+
+    path = tmp;
+
+    return path;
+}
+
 const char* getSysError(char *str, int strSize) {
 #ifdef WIN32
     int err = GetLastError();

utilsfuncs.h

@@ -57,6 +57,7 @@ bool printToConsole(const char *msg);
 char** convertToArgvArray(std::list<std::string> args);
 void addToArgList(std::list<std::string> & args, int argc, char ** argv);
 std::string findOnPath(const char* name);
+std::string resolveSymlinks(std::string name);
 bool checkDirectory(const char* path);
 void printListToConsole(std::list<std::string> l);
 std::string trimTrailingBackslashes(std::string orig);