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
Ah, got it. However, if you want to indicate to the Makefile that gflags is present, that detection happens in `build_tools/build_detect_platform`: https://github.com/facebook/rocksdb/blob/master/build_tools/build_detect_platform#L207. So you might want to add those additional parameters there, so that gflags gets automatically detected by the script