Tweak Makefile for building on BG/Q

* Omit some gcc flags not present on Power 
* Allow additional C,CXX,LDFLAGS to point to custom dependency install path
It's nothing to do with the platform, but rather with the user environment. It allows passing additional flags. I use for example
```
make -j8 -f Makefile EXTRA_CFLAGS=-I${BUILDYARD_INSTALL_PATH}/include EXTRA_CXXFLAGS=-I${BUILDYARD_INSTALL_PATH}/include EXTRA_LDFLAGS=-L${BUILDYARD_INSTALL_PATH}/lib shared_lib
```

where the install path contains dependencies such as gflags.