Compile failed with clang++ on macOS

[shinsumicco]

Compile of FBoW was failed with clang++ on macOS (10.13). I have confirmed that make operation terminated with an error with the following versions of compilers:

clang version 7.0.0 (tags/RELEASE_700/final)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /usr/local/opt/llvm/bin

Apple LLVM version 9.1.0 (clang-902.0.39.2)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

To tackle with these issues, I have added/fixed these components:

• Check _OPENMP flag is defined or not when including omp.h
• Deal with clang compiler in CMakeLists.txt
• Use rpaths on macOS to suppress warnings

Thank you for your consideration.