How to include omp.h in OS X?

I'm new in C and have some problems compiling my code in OS X.

I code Java a lot both in Eclipse and use terminal to compile my code. However now I'm learning openMP and have troubles with it.

First I downloaded Xcode to write openMP code but it didn't recognize <omp.h>. Then I installed g++. When I type g++ -v into terminal I get this:

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.3.0
Thread model: posix

But when I use g++ Mesh.cpp -fopenmp I still get

Mesh.cpp:4:10: fatal error: 'omp.h' file not found
#include <omp.h>
         ^
1 error generated.

Then I tried to install PTP into my Eclipse and got the same problem. I thought there was no omp.h in my MacBook so I searched for it and found several omp.h under folders under gcc-4.9.1/build/.

Here comes the problem. Based on the Java experience the only reason why I have the file but cannot use it is that the Class Path is wrong. However, I have no idea how to change this configuration in g++, or in Xcode, or in Eclipse. But since I can include files like <stdio.h> and compile it with all the IDEs, how can't I do the same with <omp.h>?

Another thing I noticed is that the gcc folder version is 4.9.1, but when I type gcc -v into terminal I get the same with typing in g++ -v

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.3.0
Thread model: posix

Shouldn't the version information says something about 4.9.1? Just like what java -version shows

java version "1.8.0_11"
Java(TM) SE Runtime Environment (build 1.8.0_11-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.11-b03, mixed mode)

Thanks for reading. Any help is appreciated.