cmake: renamed GFLAGS_NAMESPACE to GLOG_GFLAGS_NAMESPACE (#318)

This eliminates a conflict between glog and gflags preprocessor definition of GFLAGS_NAMESPACE that can cause the gflags namespace detection to fail (fixes #317).

cmake/DetermineGflagsNamespace.cmake

@@ -15,7 +15,7 @@ macro(determine_gflags_namespace VARIABLE)
 
 int main(int argc, char**argv)
 {
-  GFLAGS_NAMESPACE::ParseCommandLineFlags(&argc, &argv, true);
+  GLOG_GFLAGS_NAMESPACE::ParseCommandLineFlags(&argc, &argv, true);
 }
 ")
     if (NOT CMAKE_REQUIRED_QUIET)
@@ -33,7 +33,7 @@ int main(int argc, char**argv)
       file (WRITE "${_check_file}" "${_check_code}")
       try_compile (${VARIABLE}
         "${CMAKE_BINARY_DIR}" "${_check_file}"
-        COMPILE_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS}" -DGFLAGS_NAMESPACE=${_namespace}
+        COMPILE_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS}" -DGLOG_GFLAGS_NAMESPACE=${_namespace}
         LINK_LIBRARIES gflags
         CMAKE_FLAGS -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
         OUTPUT_VARIABLE OUTPUT)