```
$ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
```

Yours is slightly newer, going to open a PR to rather do this:
```
+                       CHECK_C_COMPILER_FLAG(-Wno-unused-const-variable UNUSED_CONST_VAR_WARNING)
+                       IF(UNUSED_CONST_VAR_WARNING)
+                               SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-const-variable")
+                       ENDIF()
```