fix missing gflags library

On Debian/testing and RHEL6 builds would fail due to undefined references to
google::FlagRegisterer::FlagRegisterer.  It would seem that -lgflags was
missing from the build script.

build_tools/build_detect_platform

@@ -184,6 +184,7 @@ EOF
 EOF
     if [ "$?" = 0 ]; then
         COMMON_FLAGS="$COMMON_FLAGS -DGFLAGS"
+        PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -lgflags"
     fi
 
     # Test whether zlib library is installed