For Mac-OSX
Make sure Xcode is already installed in your machine. If not, install it by executing command xcode-select --install.
- If you need latest changes download source code from repo or to get source code for a particular version download source from here 
- cd flatbuffersand execute- cmake -G "Xcode" -DCMAKE_BUILD_TYPE=Release
 - If you encounter error like - CMake Warning:
  No source or binary directory provided.  Both will be assumed to be the
same as the current working directory, but note that this warning will
become a fatal error in future CMake releases.
-- The C compiler identification is unknown
-- The CXX compiler identification is unknown
CMake Error at CMakeLists.txt:6 (project):
No CMAKE_C_COMPILER could be found.
CMake Error at CMakeLists.txt:6 (project):
No CMAKE_CXX_COMPILER could be found.
-- Configuring incomplete, errors occurred!
See also "<sourc-code-path>/flatbuffers-1.12.0/CMakeFiles/CMakeOutput.log".
See also "<sourc-code-path>/flatbuffers-1.12.0/CMakeFiles/CMakeError.log".
Inside the log file CMakeError.log:
xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance
 - then refer this SO thread
 success message is :
 - -- Configuring done
-- Generating done
-- Build files have been written to: <source-code-path>/standalone/flatbuffers-1.12.0
 
- ./FlatBuffers.xcodeprojor open project in xcode
 
- Build and Run project from Xcode UI 
- cd Debugand you can see all the binaries present.
  
 
- If you want - flatcto be available globally:
 - ln -sf <path>/flatbuffers-1.12.0/Debug/flatc /usr/local/bin/flatc
 
Tried and Tested on
MacOSX: Mojave, version: 10.14.3, 10.14.6
Xcode: Version: 10.2.1(10E1001), 11.1(11A1027)
Note: Make sure you use the same version in a project for compatibility.
If you are not sure which version to pick, then you can clone the repo and go to a specific commit and build.
For e.g. For version 1.11, Commit version is bf9eb67a.